Star Trek III: The Search for Spock
The Search for Spock begins shortly after Spock's funeral in Star Trek II: The Wrath of Khan. The damage the USS Enterprise sustained has been patched up and the ship is limping home to Spacedock, supposedly for repairs. Upon arrival at spacedock though, the intrepid crew are shocked to learn that their heavily damaged ship is scheduled for decommission. With the commissioning of the new USS Excelsior Starfleet probably feels the ship is obsolete.
Kirk's son David along with Lt Saavik have been reassigned to the USS Grissom to study the newly formed Genesis planet. The two of them beam down to the surface when they detect a life form on the surface that there wasn't supposed to be. They find Spock's burial robe in the torpedo casing that held his body, but the body is nowhere to be found.
Dr. McCoy begins behaving very strangely. He breaks into Spock's sealed quarters on the Enterprise on their way home, asking Kirk why he left him on Genesis. McCoy later attempts to charter a ship to the Genesis planet but the planet has been quarantined, largely due to the controversy over the entire Genesis project and its potential as a weapon. The attempt lands McCoy in a psychiatric ward after he attempts to give the Vulcan nerve pinch to a Federation security officer.
Spock's father, Sarek pays Kirk a visit, and also demand to know why Kirk left Spock on Genesis. Sarek explains that he believes Kirk is carrying Spock's katra and that Kirk has an obligation to return to Vulcan. Kirk replies again that he doesn't know what is going on. Sarek mind melds with Kirk and learns Spock's katra is not there. They consult the Enterprise's logs and learn that Spock had mind melded with McCoy instead and that is the reasons for the Doctor's bizarre behavior.
Kirk and Sulu break McCoy out of the mental hospital. They steal the Enterprise from space dock and Excelsior soon gives chase. however, Scotty has sabotaged Excelsior and Enterprise is able to escape.
Klingons on the scene
The Klingon Commander Kruge learns of the Genesis device, obtaining Kirk's video assessment of the project through illicit means. Impressed, he takes his Bird of Prey to the Genesis planet where they encounter and destroy the Grissom. Kruge and a handful of of his soldiers beam down to the planet to try and find the scientist behind the project. They eventually find David, Saavik and a small Vulcan boy who is aging considerably faster than normal. David believes the child is Spock's reincarnated body.
Showdown at Genesis
The Enterprise is detected by the BOP on approach and Kruge beams back to the ship and cloaks his ship. The Enterprise arrives looking for the Grissom but obviously is unable to find the destroyed ship. They do notice some kind of energy shimmer which they deduce is from a cloaked ship, which they target with their weapons. The Bird of Prey decloaks and the Enterprise fires, causing slight damage. The BOP returns fire, knocking out Scotty's jury-rigged controls and the ship is crippled. Kruge demands the surrender, and to prove he means business, orders one of the three on the surface killed, which inadvertently ends up being David. Kirk agrees to surrender but sets the ship to self-destruct and he, McCoy, Scotty, Sulu and Chekov watch their beloved ship burn up in the atmosphere with several of Kruge's crew aboard the doomed vessel. They then set off to find Saavic and Spock.
The planet begins to self-destruct. In his impatience, David used unpredictable materials in it and the planet is beginning to pay the price for it. The earthquakes cause several Klingons to fall to their deaths, leaving Saavik and Spock largely unguarded by the time Kirk and company arrive. Kruge however, gets the best of them all and wants to fight Kirk to the death. The other humans and Saavik are beamed to the BOP, where they are able to take ovver the crew-depleted ship. Kruge and Kirk fight with Kirk eventually kicking Kruge into a large lava flow. Kirk, speaking Klingon into a captured communicator, orders them beamed aboard the Klingon ship. As the BOP leaves orbit, we see the planet below tearing itself apart (in direct contradiction to the laws of Gravity no less).
The BOP flies to Vulcan where they are able to remove Spock's katra from McCoy and return it to his body. Spock is alive, although his memory is lacking.