Picture the music video for Justin Timberlake?s ?Cry Me a River?.

You remember it ? the one in which Justin glides through a sleek, modern mansion in muted shades of green and gray, exacting revenge on a blonde in a newsboy cap who looks suspiciously like his real-life ex-girlfriend, Britney Spears.

Now picture the video as a two-hour movie, with Keanu Reeves standing in for Justin. And imagine that instead of being a boy-band icon, Reeves is a chain-smoking exorcist who trolls the seedy sections of Los Angeles, babbling in a sarcastic monotone about the battle between good and evil that?s secretly being waged all around us.

And there you have ?Constantine?. It looks fantastic ? director Francis Lawrence helmed the award-winning Timberlake video and many others before making his film debut here ? but takes itself so seriously, it?s often laugh-out-loud funny.

?That exorcism wasn?t right. Listen to the ether,? Reeves says as the titular John Constantine, though he sounds more than a little like his Neo character from ?The Matrix? trilogy, to which this undoubtedly will draw comparisons.

Like ?The Matrix,? ?Constantine? exists very much in its own fully formed dark universe (the script from Kevin Brodbin and Frank Cappello is based on the ?Hellblazer? graphic novels), which you will either accept wholeheartedly or dismiss as nonsense.

Seems Constantine has always had a gift for seeing dead people ? er, demons ? who roam about but look like regular humans.

Tormented by these visions as a young man, he tried to kill himself ? and he succeeded, spending two minutes in hell, which director Lawrence depicts vividly with smoky, burnt orange intensity ? only to be revived.

Now in an attempt to earn an eventual spot in heaven (?cause suicide is a no-no in the Catholic faith), Constantine tries to rid the Earth of the devil?s disciples.

And they know exactly who he is. They come after him all the time. ?A demon just attacked me right out in the open on Figueroa,? Constantine tells a nightclub owner and longtime associate (Djimon Hounsou), sounding as if he?s quoting a Warren Zevon song.

One by one, the few friends he has get picked off, until all that?s left are his eager, fast-talking sidekick, Chaz (Shia LaBeouf from the children?s series ?Even Stevens,? providing much-needed comic relief), and police detective Angela Dodson (Rachel Weisz), who came to him for help in solving the death of her twin sister, Isabel (also Weisz).

They team up to battle bad guys who are obvious ? a flock of evil winged creatures flies toward them on the street, and when Angela pulls out her gun, Constantine deadpans, ?That?s really not gonna help? ? as well as forces who aren?t so clearly menacing. Gorgeous Gavin Rossdale, lead singer of the British band Bush, is convincingly unctuous in a chalkstripe suit as Satan?s right-hand man, Balthazar.

Also coming through with a meaty performance amid the madness is Tilda Swinton as a beautifully androgynous version of the angel Gabriel, who spars with Constantine over his desire to gain access to heaven.

Then the devil himself shows up in the form of Swedish actor Peter Stormare, dressed in crisp white, licking his lips and delivering his lines in a slightly effeminate fashion.

Is he playing this climactic scene for laughs? Or just trying to be weird for weird?s sake, as so often seems to be the tendency with this movie?

It is simply not worth going to multiplex and back to learn the answer to this or any other of the film?s ponderous mysteries.

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