There's a violent vicious alien on board our large empty spaceship. Let's all go into dark rooms alone to look for it. Oh dear, it's got me.
There's more to it than that of course, including an intriguing subplot about the venality and greed of "The Company" who want to study the Alien, and many long lingering shots of the, admittedly stunning, set design. But it's the set up of the scares that's the important thing about this movie.
As a monster movie it has few peers.
It all starts very quietly. The crew of the Nostromo, a deep space cargo vessel, are woken from hypersleep by their computer.
"Mother" wants them to investigate a distress signal on a previously uncharted planet, and Ian Holm's science officer is strangely keen on the idea.
We find out why when the investigating shore party find the remains of a huge alien spaceship. The pilot, a giant alien, is long dead, its skeleton strangely buckled as if exploded from within. John Hurt goes down into the bowels of the alien craft, and in one of the great SF scenes of all time, finds a nest of alien eggs. He foolishly gets too close to one, and it hatches, releasing a face-hugging alien that wraps itself tightly around Hurt's head, refusing to let go.
When they get Hurt back to the Nostromo, Ripley (Sigourney Weaver) is reluctant to break quarantine to let them on board, but Ian Holm's science officer overrides her, and Hurt is taken to the medical bay.
Sometime later, the facehugger seems to fall off Hurt's head and he wakes up, seemingly recovered. The crew decide on one last meal before returning to hypersleep.
So as not to spoil one of SF cinemas great shocks, I'll just say that it's about now that the alien makes it appearance, a sharp toothed monster with concentrated acid for blood and a very mean disposition.