In the final part of the trilogy, we find that space and time have been altered by the Janus Gate. Members of the crew of the Enterprise have been thrown backward and forward through time, altering history, and then returning to the present many years older or younger. Mr Spock and Chief Engineer Scott have managed to avoid the effects of the Janus Gate and are trying desperately to reorganise the crew and to find a way to pass them through the gate to restore them to their proper ages. This task is made even more difficult when they realise that they must re-adjust history to ensure not only that everyone still exists in the present, but also that the Federation itself has a future.

Captain James T. Kirk, meanwhile, remains trapped in his own past. As a boy, he barely survived a brutal massacre on Tarsus IV - an event which resonates through twenty-third century history. Can he stand by now and watch those same events unfold again? The traumatic memories of Enterprise crewmember Lt Kevin Riley -- a fellow survivor of Tarsus IV -- may be Spock's best hope to locate the captain before time runs out for them all.