A musical anomaly takes over the Enterprise and its crew sings its feelings.
James Kirk, newly made first officer of the Farragut, comes aboard the Enterprise to shadow Una just as Uhura is experimenting with a mysterious subspace fold. When she sends a recording into it, the fold creates an improbability field that makes the crew burst into song and voice what they usually keep hidden. Pike and Batel quarrel in front of the crew, Chapel admits she values her new research fellowship more than her relationship with Spock, and La'an confesses her feelings to Kirk, who is already involved with Carol Marcus.
A Klingon ship arrives intending to destroy the fold, but tests show that doing so would wipe out every starship in the fleet. Uhura works out that a single upbeat finale, in which the whole crew sings together about the fulfillment of shared work and exploration, can generate enough energy to close the field, and when the song ends it vanishes. It is the franchise's first full musical.