Risa

Start with Wrath of Kahn.

Sure. It's a sequel to a Franchise: The Movie. It is also a direct sequel to a random episode of the show. It is a self-contained story; I don't think it bothers to mention the events of The Motion Picture, it does a good job establishing the antagonist because this movie came out in 1982, it's entirely possible that even a Trek fan in the audience missed that one episode of a 20 year old TV show so there's a whole sequence where Kahn puts Chekov on his knees and recites "I am the very model of a vengeful space antagonist, I blame the death of my wife on the deeds of the protagonist. I quote from Melville's Moby Dick completely unironically. I grapple thee, I stab at thee, I spit to my last breath at thee."

Beyond beating the audience over the head with its literary references (to the point of showing a copy of Moby Dick and Tale of Two Cities on screen YOU FUCKING HACKS) it does a reasonable job of world building, and the rest of the franchise refers back to this movie a lot...to the point of remaking it twice.

It's a self contained plot that comes to an end...even though it has two direct sequels. Search for Spock is where we get fully formed modern Klingons complete with their language, and The Voyage Home is weird, but also kind of cool, and probably the most Star Trek of the TOS era movies...I also believe Voyage Home has the best soundtrack of a Trek film.