Star Trek IV: The Voyage Home - Plot

Plot

In 2286, a large cylindrical probe moves through space, sending out an indecipherable signal and disabling the power of starships it passes. As it takes up orbit around Earth, its signal disables the global power grid and generates planetary storms, creating catastrophic, sun-blocking cloud cover. Starfleet Command sends out a planetary distress call and warns starships not to approach Earth.

On the planet Vulcan, the former officers of the USS Enterprise are living in exile (after the events of Star Trek III: The Search for Spock). Accompanied by the Vulcan Spock, still recovering from his resurrection, the crew—except for Saavik, who remains on Vulcan—take their stolen Klingon Bird-of-Prey starship and head to Earth to face trial for their theft and destruction of the Enterprise. Hearing Starfleet's warning, Spock determines that the probe's signal matches the song of extinct humpback whales, and that the object will continue to wreak havoc until its call is answered. The crew uses their ship to travel back in time via a slingshot maneuver around the Sun, planning to return with a whale to answer the alien signal.

Arriving in 1986, the crew finds the maneuver drained most of the Bird-of-Prey's power, leaving it with enough for only one day. Hiding their ship using its cloaking device in San Francisco's Golden Gate Park, the crew split up to accomplish their tasks. Admiral James T. Kirk and Spock attempt to locate humpback whales, while Montgomery Scott, Leonard McCoy and Hikaru Sulu construct a whale tank for the return trip. Uhura and Pavel Chekov are tasked to find a way to recharge the ship's dilithium crystals.

Kirk and Spock discover a pair of whales—"George" and "Gracie"—in the care of Dr. Gillian Taylor at the Cetacean Institute in Sausalito, a museum dedicated to the study of whales, and learn they will soon be released into the wild. Kirk attempts to learn the tracking codes for the whales from Taylor, but she refuses to cooperate. Scott, McCoy, and Sulu trade the formula of transparent aluminum for the materials needed for the whale tank. Uhura and Chekov locate a nuclear-powered ship, and find it is the aircraft carrier Enterprise. While collecting some of its power for their ship, their presence is discovered. Uhura is beamed back but Chekov is severely injured in an escape attempt and captured. Kirk, McCoy and Taylor rescue him from a hospital and return to the now-recharged ship.

Taylor learns the whales have been released early. She gives Kirk the tracking codes but insists on coming aboard the Bird-of-Prey. They approach George and Gracie, who are being hunted by whalers. Kirk scares away the whalers by decloaking the starship in front of them. After transporting the whales aboard the Bird-of-Prey, the crew return to the future with Taylor. On approaching Earth, the ship loses power and crashes into San Francisco Bay. Once released, the whales respond to the probe's signal, causing the object to revert its climatic effects on Earth and return to the depths of space. All charges against the Enterprise crew are dropped; however, as punishment for disobeying a superior officer (as chronicled in the previous film), Kirk is demoted to the rank of Captain. The crew departs for their ship, the newly-christened USS Enterprise (NCC-1701-A), and leaves on a new mission.