Star Trek IV: The Voyage Home

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 orbit around Earth, it continues to send out a signal 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 crew of the USS Enterprise decide to return to Earth to face the punishment for the willful destruction of the ship; Spock, still recovering from being reborn on the Genesis planet, goes with them. The crew uses 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 wreck havoc on the planet until it can be answered. The crew devices 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, though their dilithium crystals are nearly drained and cannot be used in the 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 device 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. However, to be able to track them, Kirk attempts to woo Dr. Taylor to reveal the tracking codes for the whales. Despite his pleads, she rebuffs him.

Scotty, McCoy, and Sulu are able to work with materials fabricator to create and acquire "transparent aluminum", a lightweight material that can hold the water needed in the tank aboard the Bounty. While Uhura and Chekov are able to locate the U.S.S. Enterprise, a nuclear-powered aircraft carrier, and draw some of its power to restore the dilithium, but as they do, Chekov is captured, and a resulting escape attempt severally injures him and he is taken to a hospital. McCoy worries for Chekov's health due to the barbaric practices of the current era of medicine, and the crew put together an escape attempt, forcing Kirk to reveal their true intent to Dr. Taylor to pull it off. After they safely recover Chekov, Dr. Taylor reveals that the whales have been released a day early, and coerces Kirk to bring her along so she can help find them. The crew is able to locate the whales before they are hunted by whalers, and transport the creatures aboard. With Dr. Taylor and the whales, the crew reverses the slingshot maneuver and return to the future.

The Bounty approaches Earth but immediately loses power due to the alien signal. The ship splashes down into the San Francisco bay, and they are able to release the whales from the hold. The whales are able to answer the signal, causing the object to restore Earth to its normal condition and to return to the depths of outer space. Despite their success, the crew is put to trial at Starfleet for several charges, including the destruction of the Enterprise, but 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. Though smitten by 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 constructed USS Enterprise A, and depart on another mission.