After saving Earth from an alien probe in the 23rd century, the crew of the USS Enterprise, declared renegades for their defiance of Starfleet orders, decides to return to Earth and face the consequences of their actions. As they approach the planet, they find that a mysterious alien probe is transmitting powerful signals that are crippling Earth's power and atmosphere, rendering it uninhabitable. In a desperate attempt to communicate, the probe has inadvertently endangered humanity. After analyzing the signals, the crew realizes that the probe is attempting to communicate with now-extinct humpback whales, and the only solution to the crisis lies in the past.

The crew devises a daring plan to travel back in time to the 20th century to find humpback whales, which they believe will be able to respond to the alien probe and appease it. They manage to slingshot their ship around the Sun, which sends them back to 1980s Earth. Upon arrival, they encounter unexpected cultural and technological differences that challenge their mission. Adapting to the unfamiliar environment, they work to locate a pair of humpback whales, devise a way to transport them to the future, and figure out how they are going to return home, all while evading the suspicious authorities of the time. Their presence does not go unnoticed, leading to amusing and awkward encounters as they navigate the intricacies of 20th-century Earth.