With the explosion of Praxis, one of Kronos' moons, destroying almost all of the Klingons' livelihoods, the standoff between the Federation and the Klingon Empire is over. It was time for peace talks, but as it turned out, not everyone wanted peace - some were satisfied with the ongoing war.
Kirk and his loyal crew, already "retired", receive a secret assignment to conduct the first, preparatory round of peace talks with the Chancellor of the Klingon Empire. But the unexpected happens - the Klingon ship is attacked, the Chancellor is killed, the negotiations are disrupted, and the Enterprise, specifically Captain Kirk and Dr. McCoy, are blamed for this atrocity.
While the Federation higher-ups ponder how to proceed, a Klingon court sentences them to imprisonment on a segregated, ice-covered asteroid. But they don't give up hope - because they're Starfleet officers, and because Kirk has a mission - he guessed long ago that this is a plot to derail the peace talks, and gradually he begins to uncover it.
Meanwhile, the Enterprise, under Spock's command, is secretly flying to their rescue...