Star Trek: Season 2, Episode 18

2 - 18: The Immunity Syndrome

Spock senses the crew of Intrepid, a starship crewed solely by Vulcans, die. Enterprise investigates, and discovers a zone of darkness which conceals the hazard: an immense yet single-celled organism which drains energy from anything it encounters, as it did the Intrepid and her crew. And unless they can discover how to destroy it, Enterprise will be next. Worse, there are signs the organism has accumulated sufficient energy to reproduce...