An explosion destroys the bridge of the Enterprise, killing trainee crewmembers and severely injuring the main crew. Spock ignores a chunk of metal in his spine to take command and figure out exactly what happened.
His investigation soon leads him and the Enterprise's Chief Engineer Montgomery Scott to the discovery of the Tomarii, an alien race who reveres war and conflict. Despite the urging from Scott and others, Spock refuses to take the time to have his injuries treated. Scott and Spock meet with a small grouping of Romulans and Klingons, all three races having been attacked. The grouping is captured by the Tomarii, who use them in other conflicts.
Meanwhile, Spock has been framed for the explosion and Captain Kirk, recovering from his own wounds, must clear his friend's name.