Prometheus

Marked essential, and it is the most argued-about film on this list. Ridley Scott returned to the series after thirty-three years and made something barely recognisable as a monster film: no xenomorph for most of its length, a crew looking for the species that engineered human life, and a plot about creation and resentment rather than survival.

The standard complaint is fair and worth stating plainly. The scientists behave unscientifically. A biologist who has spent his career waiting for alien life runs from a corpse and then reaches out to an unknown organism as though it were a cat. Somebody removes a helmet in an unverified atmosphere. A geologist mapping the complex gets lost in it. These are not small lapses and the film has no answer for them.

What the film is interested in is elsewhere, and it is strong. Weyland is an old man who funded a trillion-dollar expedition to ask his makers for more life and is told no. David, his synthetic, has spent the voyage learning that he was built as a tool by people who will die and that he will not, and the performance Michael Fassbender gives is the reason the prequels exist at all. The opening, with an Engineer dissolving into a waterfall, is the most beautiful thing in the series.

Essential for what it starts rather than what it concludes. Covenant finishes this thread, and the pair are now the series' account of where the creature came from. Watch the two together, in order, and the second lands very differently than it does alone.