The V'Ger crisis

Around 2273 an immense cloud of energy crossed into Federation space, destroying three Klingon warships and the Epsilon IX monitoring station in its path. It was heading straight for Earth. The only ship in range was the Enterprise, just emerging from a lengthy refit. Admiral James T. Kirk took command from Captain Willard Decker, who stayed aboard as first officer, and Spock rejoined the crew after abandoning the Vulcan Kolinahr discipline.

Inside the cloud was a gigantic living machine calling itself V'Ger. It abducted the navigator Ilia and sent back a probe in her form to study the crew. V'Ger was searching for its creator and, finding Earth's carbon-based life unworthy, prepared to destroy it. The crew discovered that its core was Voyager 6, a 20th-century NASA probe that had been lost and then rebuilt by a race of machines into a vast, self-aware entity.

V'Ger sought to join with its creator to evolve further. Decker volunteered to merge with it and the Ilia probe, and together they disappeared into a new form of existence. Earth was saved, and the crisis marked the return of Kirk and his crew to active service aboard the refitted Enterprise.