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 (William Shatner) Vulcan friend, Spock (Leonard Nimoy), whose casket was launched into space and eventually landed on the planet created by the Genesis Device. On arriving at Earth Space dock,Doctor Leonard McCoy (DeForest Kelley) begins to act strangely and is detained. Starfleet Admiral Morrow visits the Enterprise and informs the crew the ship is to be decommissioned; the crew is ordered not to speak about Genesis due to political fallout over the device. The Crew members are James Doohan, as Montgomery Scott, the chief engineer; George Takei, as Hikaru Sulu, Enterprise's helmsman; Walter Koenig, as Pavel Chekov, navigation and acting science officer; and Nichelle Nichols, as Uhura, the ship's communications officer.David Marcus (Merritt Butrick)-Kirk's son, 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 "proto-matter" 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 vessel, intercepts information about Genesis. Believing the device to be potentially useful as a weapon, he takes his cloaked ship to the Genesis planet, destroys the Grissom.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 home world, Vulcan, and without help, McCoy will die from carrying the Katra. Disobeying orders, Kirk and his officers spring McCoy from detention, disable the USS Excelsior, and steal the Enterprise from Space dock 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 and Kruge immediately beams back to the Bird of Prey.In orbit, the undermanned Enterprise is attacked and disabled by Kruge. In the standoff that follows, Kruge orders that one of the hostages on the surface be executed. David 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 him beam his 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 remembers he called Kirk "Jim" and recognizes the crew.