Film, Orson Welles reminds us, is a ribbon of dreams, which not only explains some of the power of the medium but also justifies a good deal of its subject and technique. Without apparent effort cinema regularly violates the strictures of time and space, the laws of physics and biology, transgressions its audience accepts without objection and understands without instruction. Whether presenting the wildest fantasy or the most gruesome horror, motion pictures proceed through unexplored territories of the mind, showing images that viewers may not even recognize as a reflection of their own subconscious fears and desires.

One of the strangest movies of the summer, the blockbuster "Inception" examines in some exciting ways the methods and meanings of dreams. Through a considerable amount of awkward discussion, the script revisits an argument that dates back at least to Plato, questioning whether what we like to think of as reality exists only as a dream. In both its more or less contemplative moments and its many action sequences, the picture returns again and again to that central concept, constantly deceiving the audience about its action and people, who spend most of their time in other people's subconscious minds.

Leonardo DiCaprio plays Cobb, who calls himself an extractor, a man who enters the dreams of other people in order to steal some secret or, in the particular case of "Inception," to plant an idea. A Japanese businessman (Ken Watanabe) hires Cobb and his team to insert an idea into the mind of a competitor (Cillian Murphy) that will eventually destroy the competitor's business empire, a preposterous notion that exists merely to provide an excuse for all the movie's action.

Cobb and his team concoct a complicated and frequently baffling plot that involves drugging the competitor and themselves so they can all fall asleep simultaneously and enter his dream as a group. The plan challenges all of Cobb's ingenuity because it demands that he and his people must, for unclear reasons, create a dream within a dream within a dream, a hitherto untried descent into three levels of the hierarchy of the subconscious. If they fail down there, they will fall into limbo, where they may spend decades in someone else's dream (no, I don't understand it either).

Most of the action results from the conflicts between separate dreams and dreamers, so that when Cobb and his crew attempt to kidnap Murphy in the dream, a gang of armed men, projections of Murphy's own subconscious, engage them in wild chases and protracted gun battles. With the familiar illogic of dreams, some actions and contexts inexplicably change without warning or reason - a locomotive suddenly interrupts a car chase in the middle of a city street, two people climb a perpetual staircase that leads nowhere. The first moment echoes an image from Magritte, the second a drawing by Escher, both appropriate inspirations for "Inception."

Complicating the tangle of meaning, Cobb himself suffers the effects of his own history, specifically the dreams he shared with his wife, who constantly appears in his own, a projection of his guilt over her death; she frequently attempts to thwart his efforts or seduce him from his goals. Like most action movies, which exploit and overemphasize their own brand of emotion, this one establishes an overwhelming sentimentality in those repeated sequences and in its saccharine conclusion.

Cobb also instructs Ariadne (Ellen Page), a talented assistant, in the protocol of dream building and dream behavior, which provide the audience with some explanation for the remarkable action sequences that form the real substance of the picture. Given that name, Ariadne naturally constructs the mazes that create challenges to all the dreamers.

The movie's premise and the possibilities of cinema itself enable the filmmakers to achieve some remarkable moments and some spectacular effects in Cobb's fantastic journeys through the labyrinth of the mind. The grand climax of "Inception" crosscuts between three separate sequences of action, all of them connected to the central dreams and dreamers, all occurring in three different dream times, an occasionally brilliant device that might more better capture the simultaneity with a split screen. Despite its length, a surprisingly talky script, and a considerable quantity of confusion, the movie certainly qualifies as the most successful and, surprisingly, the most daring blockbuster of the season.