Two natives of the planet Cheron are brought aboard after one of them helps the Enterprise chase the other down after he had stolen a shuttlecraft from a Federation starbase. Bele and Lokai, however, have a dispute that goes far beyond a simple pursuit of a criminal. Their hatred - and, indeed, the entire shuttlecraft incident - is rooted in a deep racial prejudice which threatens to engulf not only them, but the Enterprise and Kirk's crew.