The place: Mars, during the terra-forming process. As soon as Detective Ross Sylibus arrived at the spaceport, he got involved in a murder. The victim was Kelly McCanon, the popular singer. Ross discovered something shocking: the fact that the corpse was not a human, but a "Third", a much more advanced robot than the "Seconds" which are currently in common use. Ross must team up with Armitage of the Martian Police Department (MPD) to chase the killer, Rene D'anclaude, who is destroying the "Thirds" one by one.

After discontinuing the Third serial murder cases the MPD begins investigating the random acts of terrorism by Seconds. Ross, who can't understand this decision, goes to Shinora Police Hospital to investigate D'anclaude Meanwhile, Armitage found data from Asakura which shows a young Asakura and D'anclaude together in a picture. What is the secret hidden between them? The key to solve the mystery is held by D'anclaude who's is sleeping at Hospital Shinora...

Both the original D'anclaud's and the Thirds are lost in the flames of Shinora. Now the only one remaining with the knowledge of the secret of the Thirds is Asakura. With the lead he provides them--"Asakura is at the dome on top of Dannich Hill"--Armitage and Ross head out to follow it up. But the Asakura they find there has had his memories altered, and had no recollection of Armitage. Armitage finds herself in the pits of despair... Will the secret of her existence ever be revealed to her...?