Samuel L. Jackson, who just might be the best actor working in American movies, delivers another suggestive, mesmerizing performance in "The Caveman's Valentine." The film -- an impressive second effort from Kasi Lemmons, whose "Eve's Bayou" displayed a fine instinct for narrative and complex characterizations -- is a highly unusual attempt to marry heavy dramatic material to a mystery-thriller format. While this marriage has its ups and downs, "Valentine" is an utterly fascinating film that probably would not work without Jackson.

The actor has developed enough of a following to attract moviegoers when "Valentine" opens theatrically next month. So with careful nurturing by Universal Focus and word-of-mouth reaching serious moviegoers and mystery fans, the film should far surpass "Bayou" at the boxoffice.

The film is based on a 1994 Edgar Award-winning novel by George Dawes Green, who adapted to the screen his tale of a paranoid schizophrenic in Manhattan who finds himself playing detective to solve a murder. The film is somewhat reminiscent of "The Fisher King" and George C. Chesbro's 1989 mystery novel "Bone," in which a homeless, mute man in Manhattan holds the key to a string of vicious killings.

Jackson plays Romulus Ledbetter, once a promising, Juilliard-trained musician and composer -- only his mind has deteriorated so badly that he lives in a park cave and believes a powerful foe he calls Stuyvesant monitors his every move from the Chrysler building. After Romulus, known to everyone on the street as Caveman, discovers the corpse of a young man apparently frozen to death in a tree outside his cave on Valentine's Day, he grows convinced the man was murdered.

Police dismiss his ranting. So he must fight his mental illness to pull together a convincing enough case against the man he believes is the killer, a prominent art photographer named David Leppenraub (Colm Feore in a eerily quiet, forceful performance).

To be sure, the entire story is an exercise in suspended disbelief. An audience is asked to buy, among other things, that Romulus' daughter (Aunjanue Ellis) just happens to be a cop, that Romulus' musicianship and contacts from his previous "sane" life enable him to gain entry to penthouse suites and upstate art gatherings, that a yuppie lawyer (Anthony Michael Hall) would befriend a filthy street person and that Leppenraub's own sister (Ann Magnuson) would go to bed with him.

But we do buy all these things, thanks largely to Jackson's multidimensional, charismatic acting. With his hair in dreadlocks and bulky winter clothes hanging from his body, he shuffles through city streets as if expecting an attack from his nemesis at any moment. While Romulus' illness has the upper hand, he understands he is ill. He understands he must fight with all his mental strength to hold onto what little clarity he possesses to sort out clues and interview witnesses.

Lemmons has created a visually arresting landscape to depict Romulus' "brain typhoons." Flashing colored lights that emanate from the Chrysler building roll over Romulus in waves. Scenes from his past pop up as visions filtered through a damaged mind: His wife, as her younger self (Tamara Tunie), appears to him, acting as a kind of Greek chorus to question his motives and challenge his actions.

Production designer Robin Standefer and cinematographer Amelia Vincent beautifully capture the fragmented, often frightening world of schizophrenia. Caveman is obsessed with "angels": Standefer and Vincent render these visions as images of brown male bodies turning into moths in a vaguely Moorish interior.

The balancing act between the movie's genre trappings and its stylized portrait of mental disintegration is dazzling to behold even when it doesn't quite work. In the movie's third act, it collapses completely when Romulus morphs into a combination of Sherlock Holmes and Perry Mason. But Lemmons' filmmaking has pulled us too far into Romulus' fascinating world for this to do the movie much harm.