When former time traveler John Richards is given draft orders to attend a top-secret mission at the Time Travel Academy, he has no idea that his actions will change the history of Earth as he knows it.
The mission's objective-to travel three billion years into the past and terraform Mars by diverting a prehistoric asteroid-is successful; Mars develops into an Earthlike planet. Yet no one anticipates Mars developing an advanced civilization of its own in three billion years.
This warlike civilization is bent on destroying Earth to preserve its own existence in this time paradox. John Richards must somehow correct this current timeline, yet there are forces at work in his former timeline to stop him.