In the harrowing early scenes of ''The Fan,'' Robert De Niro burrows so deeply into the soul of Gil Renard, an unstable baseball nut whose life is coming apart, that you share his panic and helpless fury, even while loathing him for the emotionally stunted bully that he is. These unnerving scenes, in which Gil is fired from his job as a salesman of hunting knives and his bitter ex-wife obtains a court order barring him from seeing his young son, are directed by Tony Scott in a style that hurls Gil's humiliations in your face like pitchers of ice water.

But before ''The Fan'' is even half over, it turns the wrong corner and goes almost as haywire as its central character. What begins as an ugly case history of an unstable loser brutally slapped around by life suddenly becomes a shallow clanking thriller that pulls the rug out from under Mr. De Niro. Once Gil begins to stalk his playing-field idol, Bobby Rayburn (Wesley Snipes), the movie starts viewing Gil not as a human being but a symbolic one-dimensional monster who has to be tracked down and shot.

Mr. De Niro soldiers on from this unfortunate turning point, efficiently shuffling through his repertory of creepy psychotic expressions, from menacingly obsequious Halloween grin to apoplectic homicidal scowl. But with no character left to play, his skillful mugging is to little avail. Diminished into the movie's garish fantasy of a celebrity stalker, he is forced over and over to proclaim variations on its one unoriginal idea: that fans of celebrities, especially the most impassioned ones, confuse themselves with their idols, and woe be the star who isn't properly grateful for the adulation.

The object of Gil's obsession is a center fielder and Bay Area hometown hero who has just signed a new $40 million contract with the San Francisco Giants. As the baseball season begins, Bobby falls into a slump that is so severe that the fans begin booing him. The superstitious Bobby blames his poor performance on the fact that a high-flying teammate, Juan Primo (Benicio Del Toro) wears his old number, 11. Bobby is so desperate to reclaim the number he is willing to buy it back, but it can't be had at any price.

Gil, who has had a couple of conversations with Bobby on a call-in radio show, empathizes so thoroughly with his hero that he takes matters into his own hands. Barging fully dressed into a hotel steam room where Juan is unwinding, he begs Juan to give Bobby back his old number. When Juan, who has had the number 11 actually branded onto his shoulder, laughs at him, Gil pulls out one of his trusty knives, and the mayhem begins.

Before it's over, Gil has kidnapped and terrorized Bobby's young son, who is the same age as his own boy. The drama screeches to a ludicrous climax in which a furious rainstorm interrupts a ball game whose life-and-death agenda depends on Bobby's slamming a home run and publicly dedicating it to his stalker.