A giant space amoeba drains the life from stars, and Spock volunteers for a likely one-way mission.
The Enterprise learns that a starship and an entire Vulcan crew have been lost to an unknown force, and then encounters a huge single-celled organism that feeds on energy. The ship is drained, the crew is weak, and Spock and McCoy argue about which of them should pilot a shuttle into the creature.
The episode combines the tension of an impossible enemy with a good-humoured argument between Spock and McCoy. Kirk must decide whether to sacrifice one of his friends. It is one of the better-regarded suspense stories of the second season.