It’s no coincidence that the poster for Constantine looks so much like that for Chinatown — faces in billowing smoke, the down-turned silhouette of each movie’s star, a sickly color scheme.

Keanu Reeves’ paranormal investigator isn’t quite in the league of extraordinary gumshoes like Jack Nicholson’s Jake Gittes. But despite its DC comic-book origins, Constantine is at heart a gritty film-noir detective story. The prerequisite gawking visuals are an afterthought and the only sign it ever was a comic book is its otherworldly plotline.

Former music-video directors don’t often inspire confidence for a debut film. But place Francis Lawrence in the company of Spike Jonze (Being John Malkovich) and Tarsem Singh (The Cell) of those who delivered whoppers their first time out.

Constantine is more commercially viable than those films, but not by much. It’s not wall-to-wall action, and when it is, its evil beastie deaths tend to have a been-there, done-Blade touch. It’s mainly a mystery movie as moody as its protagonist, moving from questions of faith to demon beat-downs to fatalist humor, all with the compelling, churning appeal of a hard-boiled B-movie ride.

It opens in Mexico, where a hapless man uncovers the Spear of Destiny, which pierced Jesus Christ’s flesh on the cross. Spear ownership has such privileges as super strength, such cons as being inhabited by a demon looking to get out and possess much more than a Mexican man’s body.

The demon begins making his way to Los Angeles, home to John Constantine (Reeves). Since childhood, Constantine’s ability to see ghosts, creatures, angels and demons has tortured him. But the chain-smoking, hard-drinking seer has parlayed it into a profession. He investigates the unknown and performs exorcisms with cab-driving cohort Chas (Shia LaBeouf, who, unlike a similar character in I, Robot, actually contributes to this story).

Angela (Rachel Weisz), a cop whose twin sister mysteriously committed suicide, consults Constantine for help. As they uncover clues, they realize the balance of good and evil (laid down in a friendly wager between God and Satan) is about to be tipped in a pretty bad direction for humans.

Kevin Brodbin and Frank A. Cappello’s script is magically aware of its characters both as fantasy film-style chessboard pieces and archetypes of the best film-noir ingredients.

The supporting cast’s pedigree helps: Tilda Swinton again uses her androgynous features to good effect as Archangel Gabriel; Djimon Hounsou delights as shaman-turned-club-owner Papa Midnite; and Peter Stormare solidifies his rep as a second-generation Christopher Walken as a wormy Satan.

But the best is Reeves, still unfairly badgered by many for “wooden” acting. As an INS agent for the spectral set, he’s a great begrudging hero with faith-based conflicts and a tragic back-story given visual oomph from Lawrence. (Scenes of an institution stint are briefly, perfectly disturbing.)

Plus, Reeves gets to toss off great two-liners (“God’s just a kid with an ant farm. He’s not planning anything.”) and establish muted, flirtatious byplay with Weisz. To boot, his weapons (including a cross-shaped Tommy gun and brass knuckles bearing crosses) are a good sight gag, too. The only distraction is his wardrobe, seemingly from a closet shared with his early Matrix character.

Sure, the intersection of double-crosses in Constantine is somewhat unimaginative (think Dogma). But because of the film’s dour humor prior to, and during, it, and the actors’ awareness, the ending works. (And what detective story doesn’t have predictable elements?) Taking the case of mimicking past detective stories invigorates Constantine far beyond a flagging comic-book movie.