An enormous space-going cloud entity feeds on the energy of whole planetary systems, and a Federation colony is next in its path. The Enterprise braves its antimatter interior and finds a way to stop it. Spock mind-melds with the entity and tells it that it is killing life, and he persuades it to return to its place of origin.

The U.S.S. Enterprise again crosses paths with Cyrano Jones, who this time has tribbles that do not breed but instead grow to an enormous size. The tribbles are back and are causing problems for the Klingons.