It's no joke for New York City cab-driver Jerry Fletcher (Mel Gibson), who sees plots everywhere he turns. This is a guy who's so distrustful of "them" that he padlocks not only his refrigerator but the individual food containers inside of it. He talks nonstop of conspiracies to anyone who will listen, and if there's no one around, he talks to himself. He compulsively buys copies of A Catcher in the Rye -- a book he's never read -- because it makes him feel secure and "normal." And every night, he brings home a stack of newspapers from which he clips articles that arouse his suspicions. From these, he concocts new theories and promulgates them via his conspiracy newsletter, which has a whopping five subscribers.

Ironically for a man who's afraid of being followed, Jerry himself appears to be stalking Justice Department attorney Alice Sutton (Julia Roberts). He spies on her through her apartment window, and he makes up excuses to visit her at the office. Unaware of the spying, Alice humors him and tolerates his rantings, partly because he once rescued her from a mugging, and perhaps also because she feels sorry for him.

Then Jerry's worst nightmare comes true: He is abducted and brutally interrogated (in a scene that's painful to watch) by sinister-looking psychiatrist Dr. Jonas (Patrick Stewart). Ostensibly working for some shadowy government agency, Jonas tries to extract information that the terrified Jerry simply doesn't have to give. Surviving this ordeal, Jerry runs to Alice, babbling incoherently, and soon she, too, is involved in the cat-and-mouse game. The pair spend the rest of the movie running from what?

In a typical film by director Richard Donner (e.g., Superman, the Lethal Weapon series), there's a straightforward battle between good and evil. But Conspiracy Theory paints everything in shades of gray. Does Jonas intend to harm Alice or to protect her? Is Jerry an innocuous kook or a sociopathic killer waiting to explode? For most of the movie, we -- along with Alice and Jerry -- don't have the full picture of what's going on. And it's never clear who's the mastermind behind the larger scenario. But apparently the filmmakers were reluctant to leave us with too much ambiguity, because they tack on a sappy ending that looks as though it was added after test audiences didn't like the original version.

Still, Conspiracy Theory has a lot going for it. For at least two-thirds of the way through, it's well-paced and intriguing, and screenwriter Brian Helgeland's script is clever and often very funny. The music was composed by Carter Burwell, who's done offbeat scores for all of the Coen brothers' movies.

But what makes Conspiracy Theory more interesting than the standard thriller is Gibson's complex, eccentric performance in what turns out to be one of the best roles of his career. Few actors would dare to make themselves as vulnerable as Gibson does here. In most of his previous films, he's played a confident, competent character who is able to manipulate circumstances to his advantage. But as Jerry Fletcher, he worries that he's not in control of anything -- least of all his own feelings. He senses there's some nameless horror inside that's driving him to behave as he does, but he's unable to articulate what it is. We can't decide if Jerry is merely a pathetic loser or downright psychotic -- and neither can Jerry.

Gibson has played crazies before, most notably Riggs in Lethal Weapon. But Riggs, though suicidal, was still firmly in touch with reality. Jerry, on the other hand, is only tenuously grounded in the here and now, and far from wanting to kill himself, he's obsessed with self-preservation. Yet for all his lunatic behavior, he never seems the least bit threatening. When he's compelled to slug someone, he apologizes afterward. Often he resembles nothing so much as a lovesick puppy, and he engages our sympathies even if we don't fully understand him.

Julia Roberts has the unenviable task of playing the straight role, and she rises to the challenge. As Alice, she's calm and patient, bringing Jerry back down to earth when he flies off on tangents. She counterbalances his giddiness by staying low-key and reserved, and we begin to sense that Alice, in her own way, is as scared of her feelings as Jerry is of his. Their odd relationship remains platonic, however, because there's no chance of a real romance between a sophisticated lawyer and a mentally screwed-up cabbie, even if circumstances throw them together.