Eleven years ago, the U.S.S. Farragut encountered a deadly cloud creature with vampire-like tendencies. James T. Kirk was on board the Farragut at that time as they entered the region of Tycho IV. The creature killed the ship's commander, Captain Garrovick, and half the crew by draining their red blood cells. To this day, Kirk feels guilt at having hesitated before firing at the creature?even though his phaser blast had no effect.

On his way to deliver necessary medical supplies, Kirk encounters what he believes is the same blood-sucking entity. He pursues it, against orders, determined to destroy it before it kills again.

A landing party beams down to the surface of Argus X, where the creature has taken refuge. In the party is the son of Kirk's former captain ... Ensign Garrovick. Garrovick spies the creature and, like Kirk 11 years before, hesitates before firing. The creature escapes and Kirk blames Garrovick for the resulting death of a crewman.

The creature leaves the planet with the U.S.S. Enterprise in pursuit. When Kirk fires on it, the creature turns and enters the starship. Fortunately, its first victim is Spock and, after tasting his copper-based Vulcan blood, flees toward its home world, Tycho IV. Fearing the creature will reproduce and, at the very least, kill others, Kirk and Garrovick prepare a trap with a matter/antimatter bomb and human blood. They lure the creature to take the bait and beam aboard the Enterprise as the creature is killed in the resulting explosion.