Continuing from 'Present Tense', the rescue mission sent out by Mr Spock in search of the missing Enterprise crew has also disappeared. Lost in both time and space, the three-man team on board the shuttle Copernicus are flung by the power of the vortex into a hellish future, where the Gorn Hegemony has all but conquered the United Federation of Planets.

Stranded on a transformed Federation colony, now a Gorn mining outpost worked by human slaves, Helmsman Hikaru Sulu encounters an older version of a man he barely knows: Pavel Chekov, leader of the scattered bands of freedom fighters against the tyrannical Gorn oppressors. Nor is Sulu himself unchanged by this new future -- in this timeline he has become a battle-scarred Starship captain.

For Captain James T. Kirk, meanwhile, the vortex has had the opposite effect, as he himself is hurled back through time and into his former boyhood...