So, after a month and a half of ho-hum summer "blockbusters," I must admit I was downright enthused by the opening scenes of Species. Here's a film that wastes no time getting down to business: A lovely teenage girl cloistered in a high-security lab watches tearfully as men in masks release cyanide gas to kill her. She's the product of a top-secret experiment, and the chief scientist (Ben Kingsley)--who is watching from an observation deck--quietly cries as she disappears, coughing, in the fumes. Suddenly, she smashes through the thick lab window. She gets away.

Within minutes, the film has already provoked an emotional response, interest in two main characters, and an adrenalizing thrill. And it keeps growing from there. As the young woman makes a Fugitive-like escape via train, doing all that she can to disguise her identity and survive, the Kingsley character assembles a diverse posse of trackers and scientists to determine her whereabouts. Named Sil, she's the genetic hybrid of human DNA and a chromosomal code sent from outer space, and the posse's mission is to kill her. But the movie plays like a fascinated game of discovery. The director, Roger Donaldson, smartly de-emphasizes the hunt in favor of focusing on the characters' sympathy for the hunted.

The posse includes Michael Madsen as a trained assassin who keeps remarking that he "respects" Sil (who, now a woman, is growing up at an accelerated rate), and the terrific Forest Whitaker as a sensitive psychic who "feels deeply" the monster's pain. Their comments give the film a dramatic push: We get a sense of Sil as not a monster but a kind of victim, or at the very least something to be understood.

Sil, who is played by the exceptionally foxy Canadian-born model Natasha Henstridge, commits a series of violent acts the posse determines are the result of her desire to reproduce. Though the movie offers a typically silly sci-fi explanation for why Sil wants to have a critter, screenwriter Dennis Feldman actually has a wonderful metaphoric subtext in mind: He's devised Sil as a fanciful representation of Woman Power. It's the best excuse yet to depict female biological impulse as something awesome, harrowing.

And Sil's self-awareness is as scary to her as it is to everyone else. When she screams at the sight of tentacles creeping out of her face before she metamorphoses into a full-grown woman, it's a heightened version of every teenage girl's apprehension about pubescent change. When she becomes obsessed with having a baby (she dreams in primal H.R. Giger imagery), she goes to a dance club called The Id and discovers the impulse to rip the spine out of any woman who gets in her way of finding a good man.

Some might call such a depiction misogynistic, but Species seems to me a dark celebration of Mother Nature. The picture says: Hey, guys, this may be your idea of an ideal babe, but don't forget--there's something serious and powerful under those perfect bosoms. Take heed, all of you, women included. (It's a wry joke that the only other female character is a naive biologist.)

Buried within the film's dippy, overstated dialogue is one gem of a line: "We decided to make it female so it would be more docile and controllable," Kingsley says.

"I guess you guys don't get out much," Madsen replies.

During most of this shrewd treatise, Donaldson keeps the action humming along nicely--his direction is invisible, always pulsing forward--and almost all of the performances are campily enjoyable. The movie peaks with a rousing French-kiss sequence that left no audience member without comment. But somewhere around the last half-hour of the movie something very disappointing happens: Species goes schizophrenic.

Because the filmmakers claw for a customory action-movie ending, they wind up destroying everything that makes Species lively. Sil's behavior suddenly lacks explanation: She begins killing indiscriminantly and her actions become ludicrous, conforming to the situational whims of the plot. All of the supporting characters switch gears too: Even Forest Whitaker, the focal point for empathy, turns into a killing machine.

Saddest of all (especially to the men in the audience), Natasha Henstridge's attractive personage disappears in favor of an H.R. Giger-designed reptile that looks like a cross between the robot in Metropolis and the woman on the back of that E.L.P. album Brain Salad Surgery--or more to the point, Alien with Bo Derek's hair. The picture ends with an uninspired sewer-tunnel chase that is like all the worst moments of Alien 3 stirred together in an oozy pudding bowl.

And then there is that last moment when you know the hero is about to blow the monster away. I could feel it coming, and I said to myself, "Please don't say a tacky death quip, please don't say a tacky death quip, please don't say a tacky death quip." But the hero raised his gun, pointed it at the monster's head, and said a tacky death quip anyway. Bang. Ouch.

Why? Why do filmmakers off to such a good start allow themselves to take the most mediocre way out? It's the most unfortunate kind of failure that begins with a vision and then turns myopic. Species is certainly engaging trash, but it could have been smashingly more--it could have been this summer's Speed or The Fugitive. And now I am left, once again, waiting...waiting...waiting....