Crossroad

Summary
From the back cover: The crew of the Nautilus, a battered starship of mysterious origin, is beamed aboard the Starship Enterprise. The group claims they are freedom fighters from the future working to save the Federation from the Consilium - a group of corrupt power seekers.

But when the Nautilus crew members suddenly seize control of the USS Enterprise, and a starship from the future arrives to arrest the renegades, Kirk must separate his true allies from those who wish to destroy the Federation.