Cast: William Shatner, Leonard Nimoy, DeForest Kelley,
The film begins three months after the events of Star Trek III: The Search for Spock as a large cylindrical object moves through space heading towards Earth, sending out an indecipherable signal and disabling the power of any vessel or station that it passes. As it takes up orbit around Earth, it continues signaling and disrupting the global power system, and then starts causing extreme weather patterns to develop over the planet while evaporating the oceans. Starfleet Command, on the last of its power reserves, sends out a subspace signal warning of the danger.
On Vulcan, the former officers of the USS Enterprise living in exile decide to return to Earth to face the punishment for the theft and willful destruction of the ship; Spock, still recovering from being reborn on the Genesis planet, goes with them. The Enterprise officers use the seized Klingon Bird of Prey, renamed the HMS Bounty, to return to Earth. As they enter the solar system, they hear Starfleet's warning and the alien signal; Spock is able to determine that it matches the song of humpback whales, long since extinct on Earth, and that the object will continue to wreak havoc on the planet until it can be answered. The crew devise a plan to slingshot around the Sun to time travel back to the late 20th century and return with a whale.
The time travel process is completed successfully, placing them in the year 1986, although their dilithium crystals are nearly drained and cannot be used in their current state to return to the future. After cloaking the Bird of Prey and landing it in a San Francisco, California park and adjusting to the time period, the crew splits up; Kirk and Spock to locate a whale, Scotty, Dr. McCoy and Sulu to devise a holding tank for the whale, and Uhura and Chekov to search for a nuclear power source to repair the dilithium crystals. Kirk and Spock are able to quickly discover a pair of humpback whales, "George" and "Gracie," at the Cetacean Institute, and are told by the Institute's whale expert, Dr. Gillian Taylor, that the whales are shortly going to be released into the wild, making the pair ideal for their needs. To be able to track them, Kirk attempts to woo Dr. Taylor to reveal the tracking codes for the whales, but she rebuffs him.
Scotty, McCoy, and Sulu are able to procure the necessary materials to construct the holding tank in exchange for revealing the formula for "transparent aluminum" to a local manufacturer. Uhura and Chekov locate the USS Enterprise, a nuclear-powered aircraft carrier, and draw some of its power to restore the dilithium, but as they do, Chekov is captured by the U.S. Navy, and a resulting escape attempt severely injures him and he is taken to a hospital. Before they leave, Dr. Taylor arrives with urgent news, and after overcoming her awe of the spacecraft, explains that the whales have been released a day earlier. McCoy worries for Chekov's safety in the hands of practitioners of barbaric 20th century treatments, and Kirk enlists Dr. Taylor's help to successfully recover Chekov. But as they are ready to leave to get the whales, she tricks Kirk into bringing her along so she can help find them. The crew is able to locate the whales before they are killed by whalers, and transport the creatures aboard. With Dr. Taylor and the whales, the crew reverses the slingshot maneuver and returns to the future.
The Bounty approaches Earth, but immediately loses power due to the alien signal. The ship splashes down into San Francisco Bay, and they are able to release the whales from the hold. The whales respond to the signal, causing the object to restore Earth to its normal condition and to return to the depths of outer space.
Despite having just saved the world, the officers are put on trial at Starfleet on several charges, including the destruction of the Enterprise. However, in light of their heroic efforts, only the charge of disobeying a superior officer is not waived, with the punishment being the demotion of Kirk from Admiral to Captain, giving him command of a new starship. Despite her feelings for Kirk, Dr. Taylor takes a position aboard a civilian science vessel, being the only expert on whales in this century. The crew departs to their new vessel, the newly-christened USS Enterprise (NCC-1701-A), and depart on another mission.
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