Okeanos, a water world settled centuries ago by a group of Federation separatists, is now rocked by a power struggle between the natives and the ex-Federation colony. When the U.S.S. Enterprise is sent beyond the Federation's boundaries to the planet to act as mediators, they are surprised to find a group of Klingons already serving as the natives' advisors.

The volatile situation rapidly deteriorates when one of Okeanos's great floating cities is completely destroyed, and Captain Kirk is arrested for sabotage. Now at the center of a deadly civil war, Mr. Spock and the crew must race to free their captain as they face the deadly secret hidden beneath Okeanos's murky depths.