There is a ship coming to earth. No one here sent it up, so it must be aliens, right? That's what Air Force Captain Anderson must think. The ship is from earth though, just not from 2024. The exploration ship Sigmet and pilot Captain Gernzer have slipped through time from 2751. The problem now is to get back home, and not change the reality they left by changing things in what is their past. There is something else going on though, and only the ship's AI knows the whole story.