Obsession

On Argus X, a landing party is attacked by a cloud-like creature that drains the red blood cells from its victims. Kirk recognises it: eleven years earlier, as a young lieutenant on the USS Farragut, he hesitated to fire on the same creature, and it killed half the crew, including his captain, Garrovick. Now Kirk delays an urgent medical delivery to hunt it, alarming Spock and McCoy. When Ensign Garrovick, his old captain's son, hesitates during another attack and men die, Kirk confines him to quarters, seeing his own past in the young man. Spock shows that their weapons could not have harmed the creature, so neither Kirk nor Garrovick could have saved anyone. The creature boards the Enterprise and attacks Spock, but his copper-based blood repels it. Kirk follows it to Tycho IV, where he and Garrovick use themselves and a supply of blood plasma as bait, then destroy it with an antimatter bomb.