The story combines science fiction with film noir--in more ways than we realize and more surprising ways than I will reveal. Its villains, in their homburgs and flapping overcoats, look like a nightmare inspired by the thugs in "M,'' but their pale faces would look more at home in "The Cabinet of Dr. Caligari''--and, frighteningly, one of them is a child. They are the Strangers, shape-changers from another solar system, and we are told they came to Earth when their own world was dying. (They create, in the process, the first space vessel since "Star Wars'' that is newly conceived--not a clone of that looming mechanical vision.) They inhabit a city of rumbling elevated streamlined trains, dank flophouses, scurrying crowds and store windows that owe something to Edward Hopper's "Nighthawks.'' In this city lives John Murdoch (Rufus Sewell), who awakens in a strange bathtub beneath a swinging ceiling lamp, to blood, fear and guilt. The telephone rings; it is Dr. Schreber (Kiefer Sutherland), gasping out two or three words at a time, as if the need to speak is all that gives him breath. He warns Murdoch to flee, and indeed three Strangers are in the corridor, coming for him.

The film will be the story of Murdoch's flight into the mean streets, and his gradual discovery of the nature of the city and the Strangers. Like many science-fiction heroes, he has a memory shattered into pieces that do not fit. But he remembers the woman he loves, or loved--his wife, Emma (Jennifer Connelly), who is a torch singer with sad eyes and wounded lips. And he remembers ... Shell Beach? Where was that? He sees it on a billboard and old longings stir.

There is a detective after him, Inspector Bumstead (William Hurt). Murdoch is wanted in connection with the murders of six prostitutes. Did he kill them? Like the hero of Franz Kafka's The Trial, Murdoch feels so paranoid he hardly knows. Rufus Sewell plays Murdoch like a man caught in a pinball machine, flipped into danger every time it looks like the game is over.

The story has familiar elements made new. Even the hard-boiled detective, his eyes shaded by the brim of his fedora, seems less like a figure from film noir than like a projection of an alien idea of noir. Proyas and his co-screenwriters, Lem Dobbs and David S. Goyer, use dream logic to pursue their hero through the mystery of his own life. Along the way, Murdoch discovers that he alone, among humans, has the power of the Strangers--an ability to use his mind in order to shape the physical universe. (This power is expressed in the film as a sort of transparent shimmering projection, aimed from Murdoch's forehead into the world, and as klutzy as that sounds, I found myself enjoying its very audacity: What else would mind-power look like?) Murdoch's problem is that he has no way of knowing if his memories are real, if his past actually happened, if the women he loves ever existed. Those who offer to help him cannot be trusted. Even his enemies may not be real. The movie teasingly explores the question that babies first ask in peek-a-boo: When I can't see you, are you there? It's through that game that we learn the difference between ourselves and others. But what if we're not there, either? The movie is a glorious marriage of existential dread and slam-bang action. Toward the end, there is a thrilling apocalyptic battle that nearly destroys the city, and I scribbled in my notes: "For once, a sequence where the fire and explosions really work and don't play just as effects.'' Proyas and his cinematographer, Dariusz Wolski, capture the kinetic energy of great comic books; their framing and foreshortening and tilt shots and distorting lenses shake the images and splash them on the screen, and it's not "action'' but more like action painting.