Spock telepathically receives the collective death cries of the entire all-Vulcan crew of the USS Intrepid, which has just been destroyed by an unknown force. The Enterprise intercepts a gigantic organism, which then surrounds the ship, beginning to cause physical and mental illness among the crew. Kirk, Spock and McCoy surmise that this paradoxically huge single-celled organism may be a "disease," as its course will soon take it through inhabited star systems. The Enterprise may turn out to be the only "antibody" capable of saving millions from the onslaught of the enormous parasite.
