An energy barrier that encircles the Milky Way galaxy, the Great Barrier caused havoc with the brains of Humans with high extrasensory perception ratings. The captain of the S.S. Valiant was forced to destroy his ship in 2069 following an encounter with the Barrier. On Stardate 1312.4, the U.S.S. Enterprise NCC-1701 also encountered the Barrier while searching for clues to the Valiant's fate. Three crewmembers were killed, and two more—Lieutenant Commander Gary Mitchell and Doctor Elizabeth Dehner—were granted almost godlike abilities before Dehner was killed by Mitchell, and Mitchell was killed by Captain James T. Kirk.