As the brave crew of the Enterprise are returning to Earth on a Klingon Bird of Prey to stand trial for the events of the previous movie, Spock determines that Earth’s demise is imminent unless they can return to 1986 and retrieve a humpback whale (which they proceed to do).

I saw this in the theater with Deb Baker and Jon Shultis during a winter trip to Pittsburgh for a small computer science education conference.