Kirk meets an old enemy, a gaseous creature that feeds off human red blood cells. He once failed to destroy it, and when his young ensign again lets it escape, Kirk becomes obsessed with hunting it down. Forced to justify his decision to Spock and McCoy, his guilt at his earlier failure is discovered. His fears are ultimately proved correct when the creature is found to be able to travel at Warp, and about to spawn. Spock's copper-based blood saves him from the entity, as he tries to stop it coming through the ventiation system. Finally, a trap is set for it with an antimatter bomb and a jar of human blood.
