Seriously though, The Voyage Home is my favorite Star Trek Movie - if not my favorite all-time movie to ever exist!
Star Trek IV: The Voyage Home, follows Admiral Kirk's crew on their way home from Vulcan, following the events of Star Trek III: The Search For Spock. Spock's katra has just been transferred to his reanimated body from Bones', and he struggles with coping with life after death. On their way home, on a Klingon Warbird, they recieve an emergency transmission from Starfleet headquarters saying to stay away from Earth due to an alien probe disrupting and essentially killing the planet.Spock is able to identify the intercepted transmission from the probe as the call of a humpback whale, known to have gone extinct in the late 20th century. The crew decide to perform a time travel maneuver to go back to the late 20th century and find humpback whales to bring to the present day to answer the probe. Upon going back in time, they fry their power crystals and have to seek power from a nuclear reactor to repower the Warbird. The crew split up into groups to complete their whale-related tasks and meet back up to save the whales from whaling, power the ship, bring a 20th century woman to the future (present day), and answer the probe, saving the world!
There's a very fun bit in the movie where Uhura and Chekov team up in search for a nuclear vessel to siphon nuclear power for the Warbird. Chekov, with his accent, goes on asking these 1980s people where he can find "Nuclear Wessels", and it's very good!
Time travel in the Star Trek Universe has always been a bit inconsistent, one of the silliest things they've done with time travel is in this movie- they brought a woman from the 1980s to the 23rd century! But, another silly is when Sulu, McCoy, and Scotty are on the task of building a tank within the warbird to house two humpback whales- Scotty ends up trading the formula for transparent aluminum for the required cut of plexiglass for the tank! When McCoy asks Scotty if they can even do that (because it would alter the future) Scotty responds with 'How do we know he didn't invent the stuff?' They both shrug off their possibly time-altering hijinks and go on with the plexiglass!