When the Enterprise is accidentally warped back in time to the late 1960s by a black star, Kirk is forced to beam the jet-interceptor pilot John Christopher (serial number 4857932) aboard. On the Enterprise (one of only 12 ships in the fleet), Christopher learns much he should not know about (like the computer which addresses Kirk as ``dear'' after having been overhauled on the woman-dominated planet Cygnet 14). Because Christopher knows the future, Kirk cannot return him to Earth. A computer analysis by Spock, however, shows that Christopher and his wife will have a son, Sean Jeffrey Christopher who will head the Earth-Saturn probe mission.
After recovering all tracking records of the Enterprise from the military (and accidentally beaming up an MP, who samples chicken soup from the food dispenser), the Enterprise warps forward in time by accelerating towards and then breaking away from the sun. As the Enterprise travels back in time, Christopher is beamed back into his jet at the exact moment in time he saw the ship and the MP is returned to the base just as he heard a noise.