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.