Kirk becomes fixated on a vapour creature that killed part of his old crew.
When the Enterprise encounters a strange cloud that smells like a creature which killed much of Kirk's former crew eleven years earlier, he becomes obsessed with destroying it. His fixation endangers the ship, and a young officer who was present at the earlier tragedy is torn between duty and fear.
The story is often compared to Moby Dick, with Kirk as an Ahab figure. Spock and McCoy question his judgement, leading to a rare moment of friction. The episode is remembered for the cloud monster that feeds on blood and for its examination of command and guilt.