The Federation starship Enterprise returns to Earth following a battle with the superhuman Khan Noonien Singh, who tried to destroy the Enterprise by detonating an experimental terraforming device known as Genesis. The casualties of the fight include Admiral James T. Kirk’s Vulcan friend, Spock, whose casket was launched into space and eventually landed on the planet created by the Genesis Device. Upon arriving at Earth Spacedock, Doctor Leonard McCoy begins to act strangely and is detained. The commander of Starfleet, Admiral Morrow, visits the Enterprise and informs the crew the ship is to be decommissioned; the crew is instructed not to speak about Genesis due to political fallout over the device.
David Marcus (Merritt Butrick)—Kirk’s son and a key scientist in Genesis’s development—and Lieutenant Saavik (Robin Curtis) are investigating the Genesis planet on board the science vessel Grissom. Discovering an unexpected life form on the surface, Marcus and Saavik transport to the planet. They find that the Genesis Device has resurrected Spock in the form of a child, although his mind is not present. Marcus admits that he used unstable “protomatter” in the development of the Genesis Device, causing Spock to age rapidly and meaning the planet will be destroyed within hours. Meanwhile, Kruge (Christopher Lloyd), the commander of a Klingon Bird of Prey, intercepts information about Genesis. Recognizing the device’s potential as a weapon, he takes his cloaked ship to the Genesis planet, destroys the Grissom, and searches the planet for survivors.
Spock’s father, Sarek (Mark Lenard), confronts Kirk about his son’s death. The pair learn that before he died, Spock transferred his katra, or living spirit, to McCoy. Spock’s katra and body are needed to lay him to rest on his homeworld, Vulcan, and without help, McCoy will die from carrying it. Disobeying orders, Kirk and his officers spring McCoy from detention, disable the USS Excelsior, and steal the Enterprise from Spacedock to return to the Genesis planet to retrieve Spock’s body.
On Genesis, the Klingons capture Marcus, Saavik, and Spock, and before Kruge can interrogate them, their ship signals that the Enterprise has arrived. Kruge beams back to the Bird of Prey.
In orbit, the undermanned Enterprise initially gains the upper hand in battle, but the Klingons return fire and disable the ship. In the standoff that follows, Kruge orders that one of the hostages on the surface be executed. Marcus is killed defending Saavik and Spock. Kirk and company feign surrender and activate the Enterprise’s self-destruct sequence, killing the Klingon boarding party while the Enterprise crew transports to the planet’s surface. Promising the secret of Genesis, Kirk lures Kruge to the planet and has Kruge beam Kirk’s crew to the Klingon vessel. As the Genesis planet disintegrates, Kirk and Kruge engage in a fistfight; Kirk emerges victorious after kicking Kruge off a cliff into a lava flow. Kirk and his officers take control of the Klingon ship and head to Vulcan.
There, Spock’s katra is reunited with his body in a dangerous procedure called fal-tor-pan. The ceremony is successful and Spock is resurrected, alive and well, though his memories are fragmented. At Kirk’s prompting, Spock recalls he would refer to Kirk as “Jim” and recognizes the crew as well. His friends joyfully gather around him.
Star Trek Command 