THE PASSENGER — *** — Jack Nicholson, Maria Schneider; rated PG-13 (violence, nudity, profanity).

What's most shocking about "The Passenger" 30 years later? Seeing Jack Nicholson at the lean, sardonic height of his youthful powers? Finding a Michelangelo Antonioni movie with an actual plot?

No, the shock is how very good this movie is. Released in 1975 to mixed reviews and audience indifference — if you went to see a Nicholson film that year, it was "One Flew Over the Cuckoo's Nest" — "The Passenger" now looks to be one of the deepest, most rigorous, and most rewarding films of its era. In a post-'60s culture increasingly obsessed with the self, the movie pulled the rug out from under its main character's very identity, asking us to consider whether a man's name or his actions outlast him. This may be the first existentialist star vehicle, and it is mesmerizing.

The movie is also as methodical as a case study (the Italian title is "Professione: reporter"), so if you have a need for speed, stay away. Your loss. Nor does it offer the antic Jack audiences came to know and love in "Cuckoo's Nest" and "The Shining." Rather, it's the brooding loner of "Chinatown" and the great, forgotten "The King of Marvin Gardens." There's no soundtrack music, either. This is a film that simply doesn't blink.

The actor plays David Locke, an American-born, British-based journalist covering a rebellion in an unnamed North African country. He has reached the end of his psychic tether — quite literally, he's spinning his wheels in the sand — and when a friendly Brit in an adjoining hotel room quietly drops dead, Locke decides to exchange personalities. The two men look similar, Robertson (Charles Mulvehill) had a bad heart and no relations — and, anyway, what does it matter who we are in the end?

So Locke thinks. The quiet marvel of the script by Antonioni, Mark Peploe, and film theorist Peter Wollen is that the character is more locked in than he realizes. Back in London, David's estranged wife (Jenny Runacre) is roused from her affair with a younger man (Steven Berkoff, the villain of "Beverly Hills Cop" back when he had hair) by the mystery of her husband's vanishing act; she dispatches David's film editor (Ian Hendry) to talk with this Robertson fellow who reported the death.

And of course it turns out that the corpse had a very vibrant life, one that involved political idealism and the risks involved