I got to watch the third Star Trek movie recently. It's a direct sequel to the second movie. With the TV show, you can watch episodes in any order. With the movie series, you have to watch them in order. It's probably for the best, that way. Giving the movie series some continuity somewhat avoids the problem of "this movie is basically a two-part episode" or "this movie just an extra-long episode".
At the end of the second movie, Spock died. But it turns out that all Vulcans have a "get out of death free" card! Convenient. When nobody was looking, Spock uploaded his brain into Dr. McCoy. We all know Spock's brain is magic, after all. So they just need to download his brain back into his body, and he'll be as good as new!
But obviously, it's not going to be that simple. McCoy is suffering from mental health issues, as a result of having Spock trapped inside him. Everyone is being retired or transferred elsewhere. And the planet where Spock was buried just got classified as Super Duper Off Limits, so no one can go there. And just for the heck of it, there's a group of renegade Klingons hanging around, waiting to kill our heroes and provide a climactic final battle.
I liked the idea of our heroes being forced into retirement. I don't want to be mean, but most of the actors in this movie look extremely old. Granted, it was twenty years after the TV show first aired, so it totally makes sense that everyone has aged. But still, it's extremely rare to see a main cast with so many wrinkles in a summer blockbuster. So the idea of "our heroes want to avoid retirement" works well with the movie. It's a solid premise to have our heroes go out for one last adventure, before they stop for good! (Although I wouldn't rule out three more movies before they OFFICIALLY stop for good.)
It helps a lot that getting the gang back together leads to the best part of the movie, where our heroes have to break into headquarters, steal their old ship, and escape before the authorities stop them. Every character gets their own chance to shine during this action sequence.
So our heroes go to Planet Genesis. The two new characters from the last film, David and Saavik, are exploring the planet for scientific purposes. And here's where things get weird! Spock's body regenerated on the planet, for some reason. Now he's a baby again. That's not the weird part. He needs to go through some extremely accelerated aging in order to get back to his normal age, right? So his age is tied to the planet's age. He and the planet get older at the same rate. This has the unfortunate side effect of him going through painful, rapid alien puberty, while the planet crumbles from accelerated growth.
So our heroes show up on the planet, at the same time as the villains. Captain Kirk and the Klingon leader have a big battle, as the planet is exploding. It looked a lot like the Frieza battle on DragonBall Z, and the special effects were pretty neat. Of course, Kirk is able to kill the villain, because there's no way that a main character is going to die in a movie that's all about hitting the reset button on a main character's death.
The end is an extended sequence, where they do the ritual of putting Spock's brain/soul/whatever back into his body. He wakes up with mild amnesia, but he remembers Captain Kirk, so I'm pretty sure his memory will be back for good by the time the next movie rolls around.