In 2286, an enormous cylindrical probe moves through space, sending out an indecipherable signal and disabling the power of ships 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 star ships 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 captured Klingon Bird of Prey vessel (renamed the Bounty, after the Royal Navy ship) and return to Earth to face trial for their actions.Hearing Starfleet's warning, Spock (Leonard Nimoy) 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 by the whales. 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. Other officers include Leonard McCoy (DeForest Kelley), Medical officer, Montgomery Scott (James Doohan) engineer, helmsman Hikaru Sulu (George Takei), Pavel Chekov (Walter Koenig), Uhura (Nichelle Nichols)Arriving in 1986, the crew finds their ship's power drained. Hiding their ship in San Francisco's Golden Gate Park using its cloaking device, the crew split up to accomplish several tasks: Admiral James T. Kirk (William Shatner) and Spock attempt to locate humpback whales, while Montgomery Scott, Leonard McCoy, and Hikaru Sulu construct a tank to hold the whales they need for a return to the 23rd century. Uhura and Pavel Chekov are tasked to find a nuclear reactor, whose energy output will enable their ship's power to be restored.Kirk and Spock discover a pair of whales in the care of Dr. Gillian Taylor (Catherine Hicks) at a Sausalito Museum and learn they will soon be released into the wild. Spock does a mind meld with a whale and figures out that it is preggers. Gillian is suspicious of Kirk and Spock, but Kirk manages to charm her and take her out to dinner. Kirk tells her of his mission and asks for the tracking frequency for the whales, but she refuses to cooperate. Meanwhile, 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, the aircraft carrier Enterprise. They collect the power they need but are discovered on board. Uhura is beamed back but Chekov is captured and severely injured in an escape attempt.Taylor learns the whales have been released early and goes to Kirk for assistance. Taylor, Kirk, and McCoy rescue Chekov and return to the now recharged Bird of Prey. After transporting the whales aboard the ship, the crew returns with Taylor to their own time. On approaching Earth, the ship loses power and comes down in San Francisco Bay. Once released, the whales respond to the probe's signal, causing the object to reverse its effects on Earth and return to the depths of space.All charges against the Enterprise crew are dropped, save one for insubordination: for disobeying a superior officer, Kirk is demoted from Admiral and back the rank of Captain where he is returned to command of a star ship. The crew departs on their ship, the newly christened USS Enterprise (NCC-1701-A), and leaves on a new mission.