Star Trek IV: The Voyage Home (1986)

The funniest Star Trek film and the friendliest to newcomers. To save Earth from an alien probe, the crew travels back to 1986 San Francisco to fetch two whales, which turns the story into a fish-out-of-water comedy about future people out of step with 1980s life, down to a run-in with a punk rocker blasting his music on a city bus. Leonard Nimoy directed it, his second film in the chair after The Search for Spock. It follows on from two earlier films, which explains why the crew arrives in a captured Klingon ship, but the opening minutes cover that, and the rescue plot itself needs no background at all. If you want to show a skeptic that Star Trek can be light, this is the one.