"Aliens" is a wow, a sci-fi war movie that gets you in its grip very early, and never lets go. In its "fasten your seat belt" storytelling, it invites comparisons to "Raiders of the Lost Ark," but "Aliens," the work of writer-director James Cameron and his wife, producer Gale Anne Hurd, goes beyond such films in the darkness of its reality and the depth of its emotion. It doesn't get any better than this.

Set sometime in the future, "Aliens" is the sequel to Ridley Scott's "Alien" -- it shares with the first movie the vicious creature that uses human hosts to spawn its young, and the heroine, Ripley (Sigourney Weaver), who was the sole survivor of the original. Otherwise, the filmmakers have wisely chosen not to compete with the previous movie's strengths. Where Scott's movie was meticulously art-directed and coldly cerebral, Cameron's is raucous, grand and big-hearted.sk

And where "Alien" focused on the creature itself, "Aliens" centers on Ripley, whom a "deep-salvage" team finds floating in space after a 56-year "hypersleep." The anonymous Company, as represented by Carter Burke (Paul Reiser), is pretty steamed that Ripley destroyed the mother ship (with the alien in it). But when the radio silence of one of the Company's colonies points toward another alien outbreak, Burke enlists Ripley in a search mission.

So she's thrown together with a company of Marines, including Vasquez (Jenette Goldstein), a burly, tough-talking woman machine-gunner; quiet Hicks (Michael Biehn) and noisy Hudson (Bill Paxton); a gravel-voiced, enigmatic android named Bishop (Lance Henriksen); Sgt. Apone (Al Matthews), a cigar-chomping top kick; and Lt. Gorman (William Hope), the group's callow and effete commanding officer.

As a screen writer, Cameron has an uncanny ear for the way these trench rats talk, their banter and swaggering bravado. He has the same instincts as George Lucas did in "Star Wars" -- make the future seem real, and lived in -- but he pushes it further. The surroundings are different, the weapons are fancier, but as the soldiers razz Lt. Gorman or ready themselves for battle, it might as well be Vietnam.

The humor is a way to get us to like these characters, so that when they're thrown into danger it's not just a cheap thrill. And it's a way to draw you into the early going, without squandering any of the cliffhangers -- it allows Cameron to pace his movie along a perfectly accelerating curve, to pack the excitement into the last 45 minutes (which is almost all climaxes) without losing the audience at the beginning.

Cameron expertly weaves his ensemble story into the main theme, which involves Ripley's conquering not only the alien, but the effect the alien has had on her, how it's made her hard-bitten. Tough, even something of a bully in arguments, Weaver's Ripley softens when she meets Newt (Carrie Henn), a little girl who is, like Ripley, the only survivor of an alien attack. In the middle of all the mayhem, Cameron finds time for some enchanting moments between these two, and the result is Weaver's best performance to date; an actress of cold, sometimes forbidding beauty, Weaver peels away that chilliness, and shows us what is warm and motherly inside her.

In her other roles, Weaver has had relatively little screen time; in "Aliens," she reveals herself to be a true movie star, who needs space to spread out, time for her understated effects to accumulate and take hold. Particularly notable in the uniformly fine supporting cast are Reiser, a deliciously out-of-sync comedian whose gestures and expressions always lag a second or two behind his words; Henriksen, who plays the android with a kind of soft, beatific purity; and Paxton, an inveterate whiner whose tough words turn to farina at the first sign of trouble.

The mission of "Aliens," of course, is to scare the pants off us, and that's where the movie succeeds most of all. Like Ridley Scott (and others before him), Cameron understands that the trick is not to show too much, to keep the monster (designed by master makeup artist Stan Winston) in the shadows; even though we already know what the alien looks like, Cameron shrouds it in mystery, so your imagination can take over. And he's a marvelously tactile director -- his movies are physical, thingy, crammed with hot, wet, claustrophobic spaces. If there's goop, he's sure to have one of his characters touch the goop -- he doesn't want you to just see his world, he wants you to feel it.

*At its heart, "Aliens" involves a myth deep in everyone's psychology, a war between a good mother and a wicked stepmother (the "Alien Queen"), and it ends with a tableau of the family triumphant, although this might be the weirdest family anyone's ever seen -- a gun-toting mom, a wild child and an android who's been sawed in half. But in that single image is the whole of Cameron's strategy -- to take what's familiar and permanent in ordinary life, and twist it, and twist it again. "Aliens," in other words, might be about a young girl who hates her stepmother and loves her mom -- it just wouldn't be nearly as much fun. Aliens, opening today at area theaters, is rated R, and contains violence and profanity.