Star Trek II: The Wrath of Khan (1982)

The best place to start, and it barely needs the show. Ricardo Montalbán's Khan comes from a single episode of the original series, but the film tells you all you need in a few lines: Kirk marooned him on a dying planet 15 years earlier, and he wants revenge. What follows is a cat-and-mouse duel between two starships, kept lean by director Nicholas Meyer, who reportedly had not seen much Star Trek before taking the job. It is also a film about getting older, about Kirk facing the cost of his own legend, and its ending lands even if you met these characters only two hours earlier.