On the first day of his new job, Tsukasa Mizugaki rushes into an elevator, where he meets a girl staring vacantly out of the window with a sad expression. He goes to Terminal Service No. 1, which is one of the offices in SAI Corp, where he is taught about Giftia. Normally, humans and Giftia work in pairs to collect expired Gifta, but Kazuki Kuwanomi, who is in charge of handling new employees, was not informed that Tsukasa was coming and thus did did not prepare a Giftia partner for him. Isla, the Giftia who met Tsukasa in the elevator previously, comes around to serve tea to the office, and is elected by the other office members to partner up with Tsukasa. The two, along with Tsukasa's trainer Michiru Kinushima and her partner Zack, go to collect a Giftia. Michiru tells Tsukasa that they are under legal obligation to collect Giftia before they expire. The group embarks on a subsequent job in which Isla is given a major role, but she fails the job, and in doing so causes a number of problems for the team. The team is then assigned a mission to retrieve Nina, a child-type Giftia owned by an old woman named Chizu. After multiple tries, Isla is unable to persuade Chizu to talk to them. Chizu overhears a conversation between Nina and Isla, during which Nina says that she feels it would be best if she is retrieved by the team. Chizu realizes she had not considered Nina's feelings in the matter and allows the team to retrieve her, but not before thanking her. Nina is then sent to the Terminal Service, where Giftia are gathered at the end of their lifespan.

Tsukasa meets Yasutaka Hanada, a spotter and a ten-year veteran at Terminal Service No. 1, whose aloof behavior surprises him. When Yasutaka asks him about how he got the job, Tsukasa reveals that his father is a friend of one of the higher-ups in the company, who decided to help him after he failed his entrance exams. Later on, after scolding Tsukasa for failing to retrieve a Giftia with Isla, Michiru vents her anger to Zack and Yasutaka before being asked by Kazuki about Isla's condition. Kazuki later takes Tsukasa out for a drink and asks him about his work compatibility with Isla, but deduces the communication problems between the two of them before even getting a response. She then apologizes for partnering Tsukasa with Isla and then expresses her frustration at ripping memories apart as part of her job. Tsukasa is then sent to the Unit Testing Room, where he meets Eru Miru and Mikijiro Tetsuguro, who are measuring Isla's physical skills. He then asks Isla about the physical training, and she tells him of her belief that she is holding him back because of all the time she spent off the field. Because of this, Tsukasa decides to do the negotiating with Giftia owners instead of Isla, in spite of the fact that it isn't what a spotter does. The next day, he begins putting together his own manual about owner negotiations and is assisted by Michiru and Zack. Once he is finished, he comes across Yasutaka, who learns about Isla's visits to the Unit Testing Room and remarks the pointless nature of those visits. When Tsukasa asks him about it, Yasutaka replies that Isla's physical capabilities are on a consistent decline that cannot be fixed, a characteristic of Giftia. Later on, Tsukasa and Isla successfully retrieve the Giftia they were previously assigned to, while Kazuki and Yasutaka talk about Isla's lifespan, which is set to expire in less than 2,000 hours, giving her less than three months to live.

Tsukasa is assigned to live with Isla in the company dormitory, a rule mandated for all marksmen and spotter duos in Terminal Service No. 1. However, he is unsettled by the fact that Isla repeatedly ignores him there. At Terminal Service No. 1, he confides to other employees about the problem, although their individual suggestions fail miserably at helping him attract her attention. Finally, Kazuki approaches Tsukasa about the problem and tells him she ignores other people during her personal time, not just him. She then tells him to take Isla out somewhere if he wants some interaction with her. Following through with the suggestion, he successfully asks Isla to accompany him to a shopping mall. There, they use the same elevator where they first met, and when Tsukasa asks her why she was using it that day, Isla replies that she had been visiting a herb shop at the mall's penthouse. They go to the shop and, to buy some more time, Tsukasa asks Isla to help him pick out some herbs as a present to a person who he unintentionally describes as being a lot like her. Afterwards, they go to a nearby amusement park, where Tsukasa admits the person he was describing was Isla herself. When he says he did this to make some memories with her about their partnership, she emotionally shuts down and tells him that she was not built to play at an amusement park, which shocks him. When Isla runs off, he purchases an item from the amusement park and gives it to her at their dorm room, telling her she can throw it away if she doesn't want it.

The Terminal Service No. 1 staff receives seven brand-new retrieval missions, and Tsukasa and Isla are assigned to retrieve a Giftia named Marcia. They are also warned of the presence of criminals who assume the identities of Terminal Service employees to retrieve Giftia and sell them on the black market. Tsukasa and Isla head to Marcia's residence, where they learn she is raising her owner, Souta, in the role of an older sister after his parents died. When Souta arrives home from school, he acts hostile towards Tsukasa and Isla, but is surprisingly willing to sign the agreement form to take Marcia away, citing that she is just a Giftia and adding that Giftias cannot be trusted in telling the truth. Unable to acquire a signature as a result of Souta's attitude, Tsukasa confides in Michiru about the encounter, and she advises him to solve the problem by having Souta believe he was truly loved by Marcia. As a result, Tsukasa, Isla, and Marcia decide to bake a cake for his birthday on the following day, and are assisted by Michiru and Zack. Afterwards, Michiru tells Tsukasa that she tried all she could to keep her father, a Giftia, from being retrieved, which resulted in him becoming a Wanderer, a Giftia that still retains its motor skills but loses its personality and memories, causing it to become instinctual and aggressive. Later, Souta returns home and is surprised by the group. Upon spotting the birthday cake, which was modeled after one used for his birthday three years ago, he remembers his family and tearfully apologies to Marcia. On the day before the retrieval, Souta is visited by a shady man, who claims to be Tsukasa and Isla's replacement from the Terminal Service and asks for Marcia.

While returning home with groceries, Marcia is suddenly ambushed by the man from the previous episode. Later, Tsukasa and Isla are contacted by Souta, who informs them about Marcia's disappearance, and they assume it is the work of a black market retriever. With only 24 hours left on Marcia's lifespan, Tsukasa resolves to retrieve Marcia and return her to Souta, and the rest of Terminal Service No. 1 joins the search. The following day, the office narrows down the search to an area where a black market retrieval service is possibly located, although their efforts are hindered by a unit from R. Security, a private security firm that was hired to assist in the investigation. Kazuki confronts the unit's supervisor, Shinonome, who gives her a map of the area under scrutiny. As the retrievers get ready to converge on the area, Tsukasa is equipped with and learns about a gun-like device designed to forcibly crash all of a Giftia's functions when they turn into Wanderers. He is then approached by Kazuki, who asks him if Isla will be able to handle the situation, but is forced to drop the subject soon afterward. Michiru later tells Tsukasa about how Kazuki tried to retrieve her father when he became a Wanderer, only for him to injure Kazuki and then be shot down by members of R. Security. Soon afterward, the black market retriever is found unconscious and it is concluded that Marcia turned into a Wanderer. Kazuki orders the rest of the retrievers to stop Marcia before she is destroyed by R. Security. Tsukasa and Isla find Marcia, only to realize Souta had been following them. Marcia then injures Isla and kidnaps Souta. Despite Isla's injuries, she and Tsukasa follow her to a rooftop, where Tsukasa nearly manages to convince Marcia to surrender. However, when Souta speaks her name, Marcia snaps and begins strangling him, forcing Tsukasa to pull out his software destruction device and prepare to shoot Marcia. However, Isla suddenly dashes towards Marcia, just as Tsukasa fires

Three years ago, it is revealed that Isla began blaming herself for not accompanying Kazuki when she retrieved Michiru's father, which wound up costing Kazuki her ankle. As a result, Kazuki retired from her position as Isla's spotter. In the present day, it is revealed that Marcia was hit by Tsukasa's device, although Isla was able to block most of the blow without being hit herself. With Isla now under maintenance for her injuries inflicted by Marcia, Tsukasa is reassigned to desk duty. After a visit from Tsukasa, Isla notes that he is still remaining optimistic despite what happened and privately questions his feelings about the situation. Once most of her maintenance is done, Isla returns to the office and realizes that she has been paying attention to Tsukasa a lot more closely lately. She confides in Michiru and Eru about it, although they misinterpret her feelings as being motivated by love and Eru decides to help Isla stalk Tsukasa. However, after multiple attempts at observing him fail miserably, Isla confesses to Michiru about her confusion on Tsukasa's unwavering optimism. Michiru then tells her that Tsukasa never forgot about the incident with Marcia, even neglecting his desk duties to go and apologize to Souta for what happened, and assumes that he is smiling out of sadness. After finishing the last of her maintenance, Isla returns to the dorm, only to find he isn't there. When night falls and Tsukasa has yet to return home, she goes to the office, where she finds Tsukasa learning that she has 1,000 hours left in her lifespan, which translates to a month. However, when given the chance to partner up with a new Giftia, Tsukasa declares that he wants to remain partnered with Isla, which makes her happy.

One morning, Tsukasa decides to ask Isla out on a date. While trying to find opportunities to do so, he finds that Isla has been doing chores for him at their dormitory and the office. After asking her about it, he learns she is trying to be useful to him, much to his chagrin. When Tsukasa musters the courage to ask her out, Isla accepts his request and decides to go to the amusement park after discussing it with Michiru and Eru, later reasoning to Tsukasa that she felt guilty about running out on him during their previous time there. However, while reading Isla's diary, Tsukasa realizes that she and Kazuki often went there when they were still partners. The next day, the two of them go there and sit at a bench, Isla's favorite spot in the park, as she was able to observe the happiness and joy of so many people, which comforted her. After learning that Isla has never tried any of the park attractions, Tsukasa takes her on a tour through the entire area, eventually ending at the Ferris wheel. As they sit inside, Isla expresses her gratitude that she is riding it with Tsukasa. As he thinks about Isla's happiness, Tsukasa faints after working too hard lately, ending the date. In the end, Tsukasa wakes up and Isla apologizes for not telling him earlier that she has 1,000 hours remaining in her lifespan. Tsukasa promises that he will stay with Isla until the end, no matter what. He then asks if she would be willing to go out with him again, to which she smiles and takes his hand. However, as he sleeps, she observes him with a doubtful expression.

During a retrieval mission, Tsukasa is surprised when the owner of the newly assigned Giftia opts to delete the latter's current personality and memories by replacing her OS and start over from scratch instead of handing over the Giftia proper, having done it several times already. Intrigued, he asks around the office if a Giftia who has gone through that experience is capable of retaining their old memories, but everyone replies there is no precedent to that. Later on, Kazuki announces that a marksman from the Terminal Service No. 3 office will be sent in, as the assigned Giftia and his owner have gone into hiding, and assigns Tsukasa and Isla to assist her. They later meet the marksman, Andie; shortly afterward, they have an awkward encounter with Eru, who mistakes Andie for her friend Olivia. After the mission is a success, Tsukasa learns from Eru that Andie is indeed Olivia, whose OS was replaced due to the company cutting costs. Later on, Eru tells Tsukasa that she reconnected with Andie and mentioned a carnival they used to go to together when Andie was still Olivia, which caused Andie to decide to go there. Not wanting to deal with her memories of Olivia, Eru tries to get Tsukasa to take Andie to the carnival instead. However, Tsukasa convinces Eru to come along, and they take Isla and Andie to the carnival the following night. There, Eru decides to let go of Olivia and create new memories with Andie, concluding it wouldn't be fair to either of them if she saw them as one and the same. Later, Isla gets lost in the crowd and Tsukasa begins searching for her, eventually finding her at a lonely walkway, terrified by exploding fireworks. After he consoles her, Isla brings up Andie and asks if her presence is hurting him. He replies that it does, but insists on continuing to be her partner. When she asks why, Tsukasa responds that it is because he loves her. Shocked and embarrassed by the sudden confession, Isla shouts that she cannot accept his love.

In the wake of being shot down by Isla at the carnival, Tsukasa has maintained a heartbroken, depressed attitude that is evident at the office. Seeing this, Michiru asks Isla about it, and she tells her that she was just confused when she shot Tsukasa down and now feels guilty about hurting his feelings. Michiru suggests having the two of them live apart for a few days so Isla could have time to sort out her own feelings, and she transfers Isla to Eru's room and Tsukasa to hers and Zack's. When Tsukasa recovers from his stupor, Michiru consoles him and tells him the reason why she joined Terminal Service No. 1. The next day, Michiru sets up a lunch between Tsukasa and Isla, during which they agree to allow Isla more time to consider her true feelings. Later on, Michiru questions the effectiveness of her treatment over the whole situation. After observing Tsukasa and Isla acting like average coworkers at the office, Michiru approaches Isla one day and asks her about her feelings for Tsukasa, and she responds after a lengthy explanation that she does return his feelings. However, she concludes from this that she must stay away from Tsukasa. When a shocked Michiru asks why, Isla confesses to her that she has approximately one month left in her lifespan. Angered by this, Michiru confronts Tsukasa about it and asks for his true intention for confessing to Isla when he knew about her lifespan beforehand; he replies that he only wishes to make memories for the both of them. When she lashes out at him for refusing to see what kind of pain will result from his decisions, he insists on keeping Isla as a partner. When he returns back to his dorm room, Tsukasa finds Kazuki already there. As Isla also arrives, Kazuki announces she intends to dissolve their partnership.

Kazuki elaborates that she is dissolving Tsukasa and Isla's partnership on the basis that romantic relationships in the office are not allowed, and intends to reassign Tsukasa to her marksman Constance while she takes charge of Isla. Though Tsukasa protests against the decision, Isla immediately agrees, later explaining that it would be better for the both of them. At the office, Tsukasa confronts Kazuki about her decision, after which she reveals she only did it to protect them both from the inevitable pain that would've resulted. However, when Tsukasa proclaims he still intends on staying as Isla's spotter, Kazuki cryptically tells him to leave the situation to her. Later on, Tsukasa and Isla go out on separate retrieval missions with Constance and Kazuki, respectively. During her retrieval mission, Isla agrees to befriend Sarah, the Giftia in question, at the request of the owner Antonio Horizon, whose lifestyle as a mafia boss made it impossible for the Giftia to live a normal life. Simultaneously, Tsukasa and Constance talk about Isla's work performance and how she had closed herself off after Kazuki dissolved their partnership three years ago. Constance then tells Tsukasa that Kazuki has faith in entrusting him with Isla. Returning back to the office, Kazuki reveals to Isla that she intentionally set her up for the mission and tries to convince her to stay with Tsukasa, saying that tearing herself away from him will only create painful memories for him. The following night, Isla muses about the unfulfilled expectations she had from her separation with Kazuki. The next day, she returns to the office, approaches Tsukasa, and explains her reason for rejecting him. Then, she proclaims that she wants to make more memories with him until the end and that she is in love with him

In the wake of starting their relationship, Tsukasa and Isla have found it difficult to talk to one another without being embarrassed. However, Tsukasa is able to ask Isla out to dinner. At the office, their approving colleagues decide to give them advice on how to appease the other. At Kazuki's suggestion, Isla asks Michiru to help her learn how to cook for Tsukasa. However, upon finding out she doesn't know any of Tsukasa's favorite foods, Isla asks him, only for Constance to suggest that the two of them just cook together. After debating on what to cook for dinner, Tsukasa suggests rice omelettes. Later on, the two travel to Antonio's manor to visit Sarah. There, Tsukasa asks Sarah what would make her happy, and she recites a sentence Isla told her during her last visit, that people are at their happiest when they are with the ones they love. Afterwards, they go to the shopping mall to purchase herbs and pajamas for Isla, after which Tsukasa admits he doesn't know what to do to make her happy. She responds that she is already happy spending time with him. Returning home, the two begin cooking rice omelettes, and although the final product was not what they were expecting, they are able to enjoy it nevertheless. Isla then allows Tsukasa to read her latest diary entries, both of which recount the days she spent with him. When Tsukasa goes to sleep, Isla adds a new entry, in which she expresses her hope that she would be able to spend another wonderful day with him.

In the middle of the night, Isla breaks down in tears and sleeps in Tsukasa's bed for comfort. The next morning, she has reverted to a bright and cheery personality, much to Tsukasa's surprise and relief. At the office, the other employees give Tsukasa tickets to a number of events that he could take Isla to. The two then use tickets given to them by Michiru and Zack to watch a romantic movie. The next day, they return to work despite having taken that day off. Following Isla's visit to the Unit Testing Room, Michiru and Eru discuss about the necessity of her increasingly frequent visits. Meanwhile, Kazuki approaches Tsukasa in private and gives him a retrieval agreement form for Isla, telling him to sign it. Overhearing their conversation, Isla decides to begin teaching Tsukasa how to raise the herbs she had been cultivating in her spare time, as well as how to make tea. While serving Michiru tea, Tsukasa is confronted by her about why he and Isla are coming to work despite taking the day off, and he responds that it was a part of their decision to carry on as they were. The following night, Tsukasa gives Isla the retrieval agreement form, and she gives him her approval in signing it. The next day, they go to retrieve Sarah, their last retrieval mission together. Before erasing Sarah's memories, Isla whispers something indiscernible to her. They then return to the office, where they find the others holding a party in commemoration for Isla's last retrieval mission. After the party, Tsukasa asks Isla what she told Sarah, and she replies that she told her of her hope that she would be reunited with the person they cherished.

It is the last day of Isla's lifespan and she and Tsukasa decide to first spend it by reading entries in her diary. They then spend the morning cleaning up their dormitory room and then taking a bath. They then head over to the office, where they take care of Isla's herbs and she leaves notes for the other employees. They are approached by an early Kazuki, who teases Isla for one last time. The two then decide to spend the day's remaining hours at the amusement park, which they enjoy to their hearts' content. Eventually, they stand at Isla's favorite bench, where she describes how she always observed the emotions of the park's many visitors and how contented they would be at the end of the day in bringing their happy memories home with them. Then, as the park closes, Tsukasa and Isla convince the operator to let them ride the Ferris wheel one last time after closing hours. There, they take turns describing what they love about each other, and Isla finally admits she loved the way Tsukasa held back his sadness and smiled, despite her worry over that characteristic. She then hands him her deactivation ring, saying that she wanted him to be the one to do it. Tsukasa begins crying, and Isla notes that it was the first time she ever saw him cry. He then puts the ring on, expresses his hope that she would be reunited with the person they cherished, and kisses Isla as her time expires. As he carries her to the vehicle, he is met by Kazuki, who thanks him for being there for her, which causes him to break down in tears. The other Terminal Service employees read Isla's letters, in which she thanks them for all the memories she had of them. In the epilogue, Tsukasa takes the same elevator where he met Isla and muses what it would be like if his lifespan was predetermined, concluding afterward that he would live that life to the fullest. Nine months later, he returns from a training course to resume work with Terminal Service No. 1, and is introduced to his new Giftia.