April is an unusually bright university student studying physics. But a holiday goes awry when the plane she's on is attacked by unknown assailants using advanced technology. She survives only thanks to the help of the man that was seated next to her, named Mark, who reveals himself to be a time traveler. He's there to stop an enemy from the future that are attempting to wipe out humanity before it can threaten them.
They discover that the FBI has been infiltrated, and enlist the help of an agent and his team to discover where the threat lies and, ultimately, stop it.
The story takes us all across the galaxy and millions of years into the future as April and Mark confront a threat more familiar and dangerous than they had expected, enlist the help of an old friend, and utilise advanced, ingenious technologies to aid them.
It also reveals disagreements in the main characters as to how time travel should be used, as well as other advanced and potentially dangerous technologies, in an invisible war waged throughout time.