Spock was the science officer aboard the Enterprise under Capt. Christopher Pike

Spock stayed with the Enterprise and was selected by Kirk to be his First Officer when Kirk took command of the Enterprise.

Pike returned to Earth for a teaching position with the Academy. Later in life, as a fleet captain aboard the training ship U.S.S. Republic, Pike suffered massive radiation damage after saving several cadets when a baffle plate ruptured in the ship's reactor. After this Spock kidnaps Pike and hijacks the Enterprise in order to return him to a planet where he can live out his life as though he was still healthy.

Lt. Gary Mitchell was a Helm officer aboard the Enterprise under Kirk. He was a close friend of Kirk, but never a captain himself.

Kirk killed Mitchell when Mitchell went mad after transforming into a god-like entity on stardate 1312.4