A recent version of this story would have hurtled toward the part where the alien jumps on the crew members. Today's slasher movies, in the sci-fi genre and elsewhere, are all pay-off and no buildup. Consider the wretched remake of the "Texas Chainsaw Massacre," which cheats its audience out of an explanation, an introduction of the chain-saw family, and even a proper ending. It isn't the slashing that we enjoy. It's the waiting for the slashing.

Hitchcock knew this, with his famous example of a bomb under a table. (It goes off -- that's action. It doesn't go off -- that's suspense.) M. Night Shyamalan's "Signs" knew that, and hardly bothered with its aliens at all. And the best scenes in Hawks' "The Thing" involve the empty corridors of the Antarctic station where the Thing might be lurking.

"Alien" uses a tricky device to keep the alien fresh throughout the movie: It evolves the nature and appearance of the creature, so we never know quite what it looks like or what it can do. We assume at first the eggs will produce a humanoid, because that's the form of the petrified pilot on the long-lost alien ship. But of course we don't even know if the pilot is of the same race as his cargo of leathery eggs. Maybe he also considers them as a weapon. The first time we get a good look at the alien, as it bursts from the chest of poor Kane (John Hurt). It is unmistakably phallic in shape, and the critic Tim Dirks mentions its "open, dripping vaginal mouth."

Yes, but later, as we glimpse it during a series of attacks, it no longer assumes this shape at all, but looks octopod, reptilian or arachnoid. And then it uncorks another secret; the fluid dripping from its body is a "universal solvent," and there is a sequence both frightening and delightful as it eats its way through one deck of the ship after another. As the sequels ("Aliens," "Alien 3," "Alien Resurrection") will make all too abundantly clear, the alien is capable of being just about any monster the story requires. Because it doesn't play by any rules of appearance or behavior, it becomes an amorphous menace, haunting the ship with the specter of shape-shifting evil. Ash (Ian Holm), the science officer, calls it a "perfect organism. Its structural perfection is matched only by its hostility," and admits: "I admire its purity, its sense of survival; unclouded by conscience, remorse, or delusions of morality."