Destruction of the Enterprise and the return of Spock

After the battle with Khan, the damaged Enterprise returned to Earth, where Starfleet ordered it decommissioned and declared the Genesis planet off limits. Sarek then revealed that Spock, before dying, had placed his katra, his living spirit, in McCoy's mind. To save both, Kirk and his officers stole the Enterprise and set course for Genesis to recover Spock's body.

On the new planet, David Marcus and Saavik, aboard the science vessel Grissom, had found a Vulcan child: Spock's body, regenerated by Genesis and ageing rapidly with the unstable planet. David admitted he had used unstable protomatter in the device, dooming the world. A Klingon commander, Kruge, seeking Genesis as a weapon, destroyed the Grissom and killed David. Facing defeat, Kirk set the Enterprise to self-destruct, luring Kruge's boarding party to their deaths, then defeated Kruge as Genesis tore itself apart.

Kirk's crew took the Klingon Bird-of-Prey to Vulcan, where an ancient fal-tor-pan ritual reunited Spock's katra with his body. Spock was restored, but the Enterprise was gone and Kirk and his officers now faced charges for their actions, which led to the events of The Voyage Home.