We are introduced to Keroro, an eccentric and blundering alien who came to destroy and conquer Earth, and is living as a slave under the roof of the Hinatas, a perfectly normal family who had lived a perfectly normal life: The mother, Aki, is an extremely busy and beautiful manga editor who rarely spends time with her teenage children; Fuyuki is the gentle-hearted and paranormal-loving son and Natsumi is the sweet but hot-tempered daughter with extremely great athletic skills, and who despises Keroro, while Fuyuki is intensely interested about him. After a few weeks of living with the Hinatas, Keroro gets his own room that is rumoured to be haunted by a ghost! However, the turn-out of the room is a big surprise for the Hinatas later on.

Momoka Nishizawa, a shy classmate of Fuyuki's who has an enormous crush on him, and has an alter ego, brings a surprise to school; Tamama, who just so happens to be in Keroro's platoon! But as we venture through this day further on, we begin to learn that there is more to than just the innocent and cute covers of Momoka and her alien partner. Momoka and Tamama are coming over to the Hinata household! Will Momoka be able to call Fuyuki a friendly 'Fuyuki-kun', instead of a formal 'Hinata-kun'? How will Tamama behave? What will turn out in the end?

Keroro becomes very, very bored because he never goes out of the Hinata household, so Fuyuki and Tamama try to figure out a way to get the sergeant to get some sunlight, without revealing his secret! And when no other plan works, Keroro learns of the perfect solution to his problem at the end! The Hinatas are feeling that somebody is watching over them when they're not in the house, and the most likely suspect is Keroro! So they secretly devise a plan to be able to catch on what Keroro does in the house when they're not there! However, the true answers prove shocking and the real suspect is revealed at the end, but it may be the last thing the Hinatas ever want in their house!

Here, we meet the serious and aggressive Giroro, and he's set deadly traps for Fuyuki and Natsumi! He's already captured Fuyuki, but can he really stand a chance against the athletic Natsumi? Is there maybe even a chance that he might... fall in love?! Giroro is very upset over the sergeant's lazy behavior, so he devises a plan to make the old, charismatic, and battle-loving Keroro come back. But then things start to get very awry, and only when a surprise guest comes to the Hinata household does Keroro really get defeated!

Keroro is targeting an old toy store to get his hands on a new Gundam model, but the thing is that it's about to be demolished! Can Keroro stop this, or will he end up learning a lesson?

Momoka invites Fuyuki to a private island for the summer as a part of one of her craziest plans to get closer to Fuyuki, but what really happens is that the Hinatas and the Keroro platoon end up arriving, too! Can her plan really work? Momoka decides that a test of courage in the island will get Fuyuki to like her, but this plan takes an unexpected turn when the ghost of the Hinata house comes to join in!

According to Nostradamus, the Lord of Terror will come and will reign over Earth in the year 1999. However, it's 2004 and the Lord has finally arrived... in the image of a Shibuya kogal high school girl. And in spite of her lateness, she still intends to fulfill her assignment and destroy Earth, along with the Gunpla modeling factory. Now it's up to everyone to either help or hinder her. Mois has been spending a lot of time with Keroro in the Hinata household, and Tamama is very jealous. So when Tamama cooks up a plan to finish her up... but could this alien that is dark with jealousy defeat someone as pure and innocent as Mois?

It's a big clean-up day in the Hinata household, but when you have four aliens helping out and one of them is a complete neat-freak, things might turn out quite interesting. Better yet, the Hinatas discover something that is almost unbelievable in the sergeant's portable refrigerator!

Mutsumi Saburo, a senior at Natsumi's school (who just so happens to be Natsumi's love interest), is trying to approach her. Meanwhile, the Keroro platoon is trying to find its fourth member, even if it means scanning through all of Japan and its giftshops! But while Fuyuki starts to have suspicions that Mutsumi could be one of the infamous Men in Black, Mutsumi comes into the household at an incredibly fast pace, and his entrance is also completely suspicious. Is Mutsumi really a Man in Black, or is he the partner of someone incredibly special? Aki is turned into a teenager by Kururu's new weapon - What If I Can Do Over My Life gun - and takes the opportunity to revisit her youth by going to her children's school. However, in Fuyuki's uniform, she is harassed and soon causes trouble for her son, as everyone thinks she is him. In the meantime, Keroro must find Aki, within 3 hours, or else the damage will be permanent.

Keroro has a cavity and it is up to the platoon and the Hinatas to battle the enemy invaders in his mouth with Kururu's invention, with a slight parody of Gundam.

The Sentimental Consultans arrive to film Keroro Platoon on earth. These celebrities are less than desirable characters when they try to take advantage of the platoon and the Hinatas.

Sumomo flees her constraining life as a Space Idol by going to Pekopon where no one recognizes her. However, she realizes that she does enjoy being a celebrity and performing for all her fans. Giroro, the most dangerous man in the universe, has his belt taken. Without it, he is useless as he can't even keep his balance. Neko saves the day when she finds the belt and prevents Giroro from leaving Pekopon and Natsumi. The belt was revealed to contain a picture of Natsumi.

As other alien invaders on earth are being exposed, the platoon becomes concerned for their own security. Soon an intruder is discovered, but she turns out to be Natsumi's new classmate and the friend of the missing Zeroro. Disappointed and feeling abandoned all those months, the new Dororo breaks off relations with the platoon. In the house, Keroro reflects back on how he, Giroro and Zeroro were childhood friends and included some of the trauma he caused. In the meantime, Dororo and Koyuki have been kidnapped by Viper. The platoon attempts a rescue operation, but is quickly defeated by Viper (who is much stronger than the incomplete platoon). A remorseful Keroro wholeheartedly apologizes to Dororo for breaking his prized music box and awakens him. The now complete platoon, using an energy ball defeats Viper. However, the shock of knowing that Keroro broke the music box greatly upsets Dororo and causes him to reconsider joining the platoon.

To win back the nature loving Dororo, flowers are planted throughout the city. Though Natsumi is suspicious that Keroro might be up to his old tricks, all the flowers bloom and everybody is happy. Unkowing the garbage men take away all the flowers and everybody is depressed Natsumi feeling bad for the others begs them to have one more plant left, but the flowers then are taken away by Momoka. The next day Natsumi secretly plants one flower unknowing the flower is really the mutant plant starts hurting Natsumi, well maybe next time Natsumi shouldn't be so hard on Keroro!

In a freak lightening accident, the violent aggressive Momoka is spun out of the usually sweet and docile one. The spiked hair Momoka using the vulgar male pronoun "ore" takes over the base and forms the new "Momoka Platoon" and starts the invasion of earth. Defeated, the inept Keroro goes into hibernation. Just as the violent Momoka is ready to push the button to launch the missile, the docile Momoka arrives in time to stop her other self from destroying Earth. They agrees to join back together in one body and will one day declare their love for Fuyuki. Keroro, revived, offers to join them through synchronized swimming. After a wonderful performance, a complete Momoka emerges from the pool. However, another complete form also emerges—the Keroro-Giroro combination.

Members of the platoon and their human allies witness Mois beating up people and taking their money. In disbelief, they try to find clues to Mois's secret activities, but not even her videogram to her father yields any. Hurt that no one believes her innocent and at the same time embarrassed to tell them of her activities, Mois runs out. In pursuit, they encounter a delinquent Mois getting ready to harm some of Natsumi's classmates. To the rescue, Natsumi fights Mois, until another Mois appears. Through a flashback, Mois reveals that she modelled herself after Asami, a brave girl who had once defended the weak. Now Asami has become a delinquent and they must intervene to bring her back from her lone wolf days. Through Kururu's invention which generates holograms, Fuyuki's script, Mois's acting, and everyone else's assistance, they manage to have her return home.

It's summer, and the school swimming pool is open for use. While Fuyuki has been dreading this all year (he can't swim), Keroro hatches a new scheme to infiltrate the swimming pool and have some fun on his own, resulting in a confrontation with Natsumi in the pool, where Keroro's amphibious advantage over her is exploited to the fullest. After the historic Earth versus Keron Swimming Meet, a second interplanetary competition begins at nighttime, as humans and Keronians duel each other with the scariest ghost stories they can think of. While Fuyuki's little-known talent for ghost stories wins the day, the only real loser here is Giroro, who gets jerry-rigged to a meter and subjected to all the scares (and some physical violence) from both sides.

It's time for another important summer ritual, the family trip to Doinaka Beach, but Hinata Aki is uable to accompany the children due to work commitments, leaving Natsumi and Fuyuki without an adult guardian, until Kururu reveals his latest invention, an age manipulation beam that turns Natsumi into a full-grown adult. An unexpected side effect causes Natsumi to utter 'old-man' jokes involuntarily, while Keroro's hidden angle and true reason for infiltrating the beach is revealed—the super-rare 'Dangale' knockoff model kits, offered as a prize for entering the Swimsuit Beauty Stand-Up Comic Competition.

Low on invasion funds, the platoon takes on food vending and gaming jobs at a festival. Keroro's unscrupulous manners are challenged by Natsumi and they face off at a turtle fishing game. Defeated, the frogs are ready to leave the festival, until Natsumi told them that as long as they are legitimate, it doesn't matter who attends the festival. Stirred by her compassion, Giroro dashes to stop Keroro trump card, but instead find himself, Keroro, and Tamama sent into the air as part of the night's fireworks. The red and green are exceptionally beautiful, but no one can see the dark blue color of Tamama. Witnessing the power of radio, the platoon, minus Dororo, set out to jack all the stations and put on their own programming. Different and novel at first, the radio progam gains a huge draw. However, a poorly impersonated Sumomo-chan by Giroro quickly reverses that and soon listeners turn off their radio one by one.

The trip to the beach that Aki promised the children in episode 18 gets put off again due to car trouble, but living with aliens proves to have its advantges, as Natsumi finds out about the Keroro Platoon's high-tech submarine, and begs them to take her to the beach, putting a slight dent in Keroro's newest scheme, to conquer the world through its oceans. Fuyuki comes along, hoping to gather some oceanic research to complete a school project, but winds up meeting a mysterious girl. The girl's true origins are hinted at as the Keroro Platoon's undersea flag-planting activities are met with substantial opposition—the ancient Nontolma civilisation, already angered by humanity's pollution of its oceans, and looking to make an example to the Keroro Platoon.

Keroro engineers some falsified invasion footage to send to Keron high command (and make some quick cash on the side) but a sudden city-wide power outage leaves this newest scheme in ruins, as Kururu conveniently forgot to save his progress. Also feeling the heat (literally) is Natsumi, without air-conditioning to beat the rising summer temperatures. Koyuki leaps to the rescue with some traditional summer rituals of her own, while Keroro's backup plan for the next power outage is less than successful. Aki's car trouble has passed, resulting in a brief glimpse at her unique driving skills as the Hinata family (and three Keronians stowing away in the boot) take another field trip, to the countryside this time. Giroro runs off to do some survival training in the forest; Keroro joins Fuyuki for a round of baseball, but gets chased into the forest by a huge dog; Tamama engages a seemingly normal rhinoceros beetle in a martial arts duel; and we meet Grandmother Hinata, who not only learns about the Keronians on her own, but secretly lends them a hand.

Just as a complacent Keroro is happy being unchallenged in his position as the leader, a flying arrow message is shot from HQ: "Tamama is captain." Keroro and the others are speechless at first, but following orders, Tamama becomes captain. Power corrupts Tamama and he soon turns into a monstrous tyrant by banishing anyone who disagrees with him, even his most beloved Gunsou-san. What will now happen to Keroro?

Keroro steals the Kero Ball from Fuyuki's room, and accidentally clones hundreds of himself, all just as lazy as the original. Kururu's attempt to rectify this situation turns them into mindless drones under Keroro's control, but just as this new power starts corrupting him, Kururu discovers that the malfunctioning Kero Ball may pose a bigger threat to Keroro himself.

Space Detective 556 (Kogoro) and his sister Lavie formally debut in this episode. Keroro takes on this new adversary single-handedly—in a harmless, friendly tussle, as it is revealed that Keroro and 556 are old friends. Life for interstellar immigrants is not an easy one: 556 is unable to hold down a proper job (his habit of collecting junk doesn't help), making Lavie the sole breadwinner - until Keroro tracks down the perfect job, allowing 556 to do what he does best.

This episode marks the formal debut of Baio Nishizawa, Momoka's father and head of the Nishizawa Group. When Momoka refuses to join her father in Scotland in favor of spending more time with Fuyuki, Mr. Nishizawa flies to Japan to meet with her instead, which gives Keroro the idea of pitting Keron's invasion force with the world-superpower-like Nishizawa group. When Momoka's current bodyguards fail to deter the superior forces of Nishizawa's black ops brigade, Keroro formally intervenes, and the first true interplanetary conflict begins, as Keron's invasion forces go toe to toe against Earth's greatest military superpower.

The show shifts its focus back to Japanese culture this week, as we witness the autumn School Sports Meet - yet another thing Fuyuki has been dreading all year (see episode 17). Natsumi on the other hand is excited about running in the three-legged race with her mother Aki - until she learns that work commitments have held up Aki yet again. Keroro schemes to win Natsumi's gratitude by infiltraiting the sports meet with Giroro and stalling the events until Aki can make it, while Tamama and Dororo head to Aki's office and attempt to speed up her work - but both efforts seem doomed to failure, forcing Giroro to act on his own.

Keroro receives a letter from his father, rumored by many to be Keron's most fearsome soldier and commander, now planning to visit Earth. Keroro is forced to engineer a massive coverup for his lackluster 'invasion' of Earth, or risk his father's wrath. Keroro's newest scheme involves creating his own onsen, by digging a new hot spring from the Keron underground base. After drilling underground in the Magma Swimmer drill tank, the Keronians discover buried alien relics left behind by an ancient invasion force, and chaos erupts as the ancient machines reactivate on their own and attack the Keronians - the ensuing battle leaves the Magma Swimmer exhausted of power, and the Keronians without any way to return to the surface...

The platoon, having never experienced snow, joins the humans in a snowball fight. To make the game more interesting, they designate hostages and a time limit to capture the enemy flag and free their comrade. Paul and Giroro get caught up in the game while everyone else has moved onto building snow creatures. The narrator's reading of the script upon introducing Kururu makes him want to prove how much of a bad ass jerk he really is. For each victim, he devices a unique plan. All attempts to reason with him fails, even threats of violence. However, it's Mois' pure and innocent heart that finally cracks the depressing jerk. He can't stand her shiny innocent eyes.

It's time for the school drama club's annual production. Natsumi's two recurring classmates Yayoi and Satsuki beg her to help them in their stage production of Peter Pan, and while Koyuki relishes the idea of performing alongside Natsumi, it turns out that stage acting is one of the few things Natsumi is completely hopeless at. Keroro offers his assistance yet again, subjecting Natsumi to all forms of stage training - but just as she gets her act together, the sight of Saburo in the audience watching her performance reduces her to the same hopeless state she began in. Keroro intervenes in full view of the audience, and rapidly escalating chaos ensues. Also in the audience was one of the three new characters debuting - the Kissho Academy Newspaper Club aka KGS. It turns out that they have been carrying out their own investigations on every one of the Keroro platoon's school infiltration shenanigans, down to Fuyuki's connection to these alien invaders, and they plan to break into the Hinata household to search for evidence - but Keroro, now grounded and barred from Gunpla-shopping after being spotted by the KGS in public, has every intention of taking the battle to them.

We meet Taruru, one of Tamama's martial arts students, who lands on Earth to visit his number one idol Tamama - who's been secretly feeding a pile of hogwash to the boys back home, embellishing his own image while deflating everyone else around him (except Keroro). Only after Taruru leaves do the three remaining Keronians, the Hinata children, Paul and Momoka gather around Tamama and exact some overdue justice. Momoka hatches a plan to make herself more physically attractive to Fuyuki, and Tamama enlists Keroro's assistance in this matter. A large number of Kururu's inventions get showcased here (and tested on Giroro, with painful results), but just as Momoka loses all hope, she chances upon something new - Hinata Aki DNA Pills, which prove a little too effective... and not just on Momoka...

A restless Keroro hops on his new space motorbike KRR-SP and takes off... His bike breaks down and strands him in the middle of nowhere. Cold, hungry and lonely, Keroro wants to go home, but can't. He doesn't even know where he is, except in the mountains. It's now up to Natsumi to go out and bring her stupid frog back home.

Keroro's latest scheme involves recruiting Earth's animals into his invasion army, with the help of Kururu's 'We All Live Together' beam gun that turns animals into humans. The scheme predictably backfires, as none of the animals are interested in joining Keroro at all - even the mighty Bengal Tiger, spoiling for a fight from the moment he is turned, is against the idea of going to war. When Keroro unwittingly reveals his intention to replace the Keroro platoon with animals, the normally loyal soldiers stand aside as Keroro raises more animals than he can lay down. The cat that Giroro brought in from the rain has now become his most loyal companion, so much that it's started to express great jealousy in Natsumi's presence. When a distressed Giroro wants to give Natsumi a pinecone pendant but can't, the cat feels his distress and wants to help, but also can't... until the 'We All Live Together' beam gun winds up in its paws.

Witnessing the merchandising power of the 'Admiral Geroro' anime, Keroro hatches a new scheme to conquer Earth throuugh the creation of his own anime - but the actual process, from storyboarding to artwork to animation, prove far too troublesome and frustrating for the Keroro platoon to handle. They pay a visit to an actual animation studio, picking up the tricks of the trade, and after returning to work, the final product is ready for viewing - but Keroro decides that more work is needed, and returns to the research & development stage (the fact that said final product turned out to be quite horrible isn't even brought up).

After experimenting with several types of fortune-telling, Momoka learns that her romantic destiny with Fuyuki will be realised in an onsen. Thus she engineers a scheme of her own, allowing Fuyuki to 'win' an onsen trip for the whole Hinata family and one guest... problems arise when Keroro and Koyuki have their minds set on that guest ticket at the same time. While Keroro is easily dispatched with the task of solving a Rubik's Cube, Kururu forces Momoka and Koyuki to battle each other for the guest ticket, and tradition clashes with technology as Koyuki pits her ninja skills against Momoka's new robotic battlesuit! Fuyuki is late for school, while Keroro is fleeing Natsumi's wrath after breaking her mug - the two meet each other on a shortcut to school. With Mutsumi standing in for the narrator, the simple act of walking down this seldom-used lane, filled with old memories and new discoveries, turns into an evening to remember.

Natsumi's birthday rolls around this year, and Keroro has big plans for a surprise party. However, Fuyuki has doubts in this plan; not least because Natsumi told him that she thought parties were childish. So, will the party work?

Winter looms and temperatures take a dive - Keroro is introduced to Fuyu-Shogun, the General of Winter, which is really just a term given to the cold air currents originating from Siberia, but Keroro visualises Fuyu-Shogun as something resembling Demon Emperor Arago from Yoroiden Samurai Troopers. When the simple act of stepping out the front door proves too much for Keroro, he decides to go through endurance training to combat the cold front, and subjecting the Hinata children to the blizzard simulation against their will only shows them to be far more resourceful than Keroro was. When Tamama tells Momoka about the blizzard simulation and the accidents that followed, Momoka offers to let the Keroro platoon continue training on her private snowy mountain resort - with the real intention of getting closer to Fuyuki, as usual. While the Keroro platoon starts trekking up the hazardous snowy mountains, Momoka puts her own plan into motion, activating the mountain resort's snow generators - but Kururu chooses to activate his private satellite's weather manipulation function at the same time, and the mountain resort is plunged into an artificial ice age.

Keroro's latest scheme involves making Dororo give ninja lessons to raise money for the platoon. When nobody actually signs up for said lessons, Keroro makes the rest of the platoon learn Dororo's ninja arts instead - but none of them seem to be serious about learning anything. To make matters worse, 556 sees their ninja lessons as competition against his own attempt to give Space Detective lessons, and tries to shut the operation down. After Natsumi's radio show was interrupted by 556's invasion, it gets interrupted yet again by a pterodactyl flying into her room - it turns out that Keroro, inspired by a certain Hollywood movie, is using Keronian technology to clone dinosaur DNA and open a theme park featuring said dinosaurs. Using Kururu's mind control apparatus, Keroro has tamed a tyrannosaurus into doing his bidding, but an accident causes the mind control cone to come off the T-Rex's head (and onto Giroro's head), causing it to run wild through the underground base.

As part of a galactically-broadcast Candid Camera-esque program, Keroro plays a prank on Giroro, with the help of a Pekopon suit modelled after Natsumi, with Sumomo at the controls. Hilarity ensues as Giroro distracts himself from faux-Natsumi's advances with physical training and weapons maintenance - causing an exasperated Sumomo to attack Giroro, and incurring the wrath of the real Natsumi when she sees her own towel-clad self leaping across town. When she opts to stop them on her own, Kururu gives her his latest invention - the Natsumi battlesuit. Natsumi finds herself stuck with Kururu's battlesuit, which has a tendency to activate when anyone nearby utters something even remotely sounding like the attack command. When the invade command kicks in, the battlesuit AI takes over Natsumi completely, causing her to destroy anything the Keroro platoon sends after her. Giroro is forced to confront her as well, and Kururu gives him the battlesuit deacivation code... "Natsumi, my love. Kiss me tender and hold me tight, forever!"

Keroro hatches a new scheme for the Christmas season, but Dororo and Tamama refuse to go with it for different reasons. Tamama tells Keroro about Santa Claus, who knows when you've been naughty or nice, prompting Keroro to carry out investigations on Santa Claus. Unfortunately the Christmas season of cheer and good will begin to take effect, as Keroro's remaining loyalists Giroro and Kururu desert him as well. Left to his own devices, Keroro begins to discover the Christmas spirit in a way he hadn't really expected. Keroro is unusually hot-blooded about spring-cleaning time, taking charge of housecleaning operations in the Hinata household, and imparting valuable tips and tricks to the Hinata family. At the end of the day, human and Keronian alike sit down for noodles, listen to the bells, and welcome the new year.

It's the first episode of the new year - and the Keroro platoon's New Year celebrations involve subjecting Giroro to different forms of abuse for some reason. While discussing new year resolutions, Angol Mois is reluctant to reveal hers, until a reaction in her alien physiology, triggered by Keroro's suspiciously glowing 'red bean soup', causes her to go completely berserk. The Keroro platoon receive some mail from well-wishers at home, the Keronian children - and at least one reminder from Keron high command, ordering Keroro to submit a large amount of overdue homework immediately. With the help of his loyal troops, Keroro overcomes every possible distraction and completes the homework on time... but one last obstacle looms on the horizon...