Star Trek IV: The Voyage Home

Essential, and the most successful of the original films commercially for a reason: it is a comedy, it has almost no action in it, and it is the entry that people who dislike Star Trek tend to enjoy. A viewer can start here cold and follow everything.

The premise is absurd on paper. An alien probe arrives at Earth transmitting a signal that nobody can answer, because the species it is addressing has been extinct for two centuries, so the crew travel back to 1986 San Francisco to collect two humpback whales. What the film does with that is spend ninety minutes on fish-out-of-water comedy, with a Vulcan trying to swear, a doctor appalled by twentieth-century medicine, and a starship captain explaining away his colleague's behaviour by claiming he took too much LDS in the sixties.

Nimoy directed again and the lightness is deliberate. Two bleak films in a row had earned the crew a holiday, and the film is openly affectionate toward them in a way the franchise rarely permits itself. It also carries its environmental argument without preaching for most of its length and then preaches for about ninety seconds at the end.

It closes the Khan trilogy and resets everything, so it is the natural place to stop if you want the original crew's story complete. The two films after it are optional in a way these three are not.