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This movie is vaguely based on the comic book series "Hellblazer," which I haven't read, so I can't compare the movie to the source material. But I will say that while I usually put reviews of movies based on comics (even those I haven't read) in a section specifically for such films, there are occasions when I feel the story's genre outweighs the format of its source material... and this is definitely a supernatural movie. Anyway, the movie starts with some text which lets us know about the Spear of Destiny, which would allow he who possesses it to control the fate of the world, and which has been missing since the end of World War II. (As soon as I read that I was like, "Oh yeah, didn't Hitler have that for awhile?") Not that I know a lot about it, but I did know, as was mentioned later in the movie, that it was the spear that killed Christ when he was on the cross. In any event, after the little bit of text, we see some guy in Mexico dig up the spear, wrapped in a Nazi flag. I have no clue how it ended up in Mexico, but whatever. He takes it and just starts... going somewhere.
Meanwhile, the majority of the movie is set in Los Angeles, where there's an exorcist named John Constantine (Keanu Reeves). A case he's working at the start of the movie gives him more trouble than expected. It seems as if a demon was trying not just to possess a girl, but to use her as a gateway to physically escape Hell into our plane of existence. We later learn that there's a rule that full-fledged demons cannot leave Hell and full-fledged angels cannot leave Heaven, but there are half-breeds of each type that can exist on our plane, and can influence human beings in the name of a "standing bet" between God and Lucifer. We also meet a few of Constantine's associates, including Beeman, Hennessy, and Chas (Shia LaBeouf). We don't actually see that much of any of them in the film, but they're all important in their own ways. Chas was possibly my favorite character in the movie, a kid who mostly serves as Constantine's driver, though he's supposed to be his apprentice. In spite of his frustration throughout most of the film at his mentor's refusal to involve him directly in his cases, in the end Chas proves quite helpful. I should also mention a man named Papa Midnite (Djimon Hounsou), a witch doctor who now runs a nightclub, which is neutral ground in the war between Heaven and Hell.
At the same time, a woman named Isabel Dodson, who was in a psychiatric ward, jumps off the roof and kills herself. Her twin sister Angela (Rachel Weisz), a detective with the LAPD, refuses to believe it was a suicide. After coincidentally crossing paths with Constantine a couple of times, she later goes to him to ask for his help, believing Isabel had been brainwashed into killing herself. Both Isabel and Angela are Catholic, and Angela is troubled that because her sister's death was a suicide, she can't be given a proper burial by the church. We learn that ever since Isabel was young, she claimed to see things that other people couldn't, which led her to get involved in the occult, and which also led to her being institutionalized. Angela has trouble accepting the existence of the supernatural things her sister believed in (she claims not to believe in the Devil, so I have to assume she's a lapsed Catholic, and her desire for her sister to be buried by the church was strictly for the sake of Isabel's beliefs, not her own.) Still, she's soon forced to accept that it is all real, after all, not just because her new association with Constantine is putting her in the middle of all the mysterious events which are transpiring, but because she turns out to be important to the plans behind these events, herself.
I don't really want to say too much about what the reason is behind everything that's going on in the movie or what it all leads to, except that it involves a half-demon named Balthazar, a half-angel named Gabriel (Tilda Swinton), and Mammon, who according to the film is Lucifer's son (though we don't actually see Mammon, his plan is integral to the plot). Another important aspect of the story is that Constantine himself had committed suicide in his youth, and though only dead 2 minutes on our plane before being revived, his soul spent a lifetime in Hell. In the 20 years since then, he's been sending half-demons back to Hell, hoping to buy his way into Heaven. Though apparently, this won't work, since all his efforts are ultimately motivated by this selfish desire; to escape the fate of returning to Hell when he eventually dies, he'd have to do something truly self-sacrificing. Anyway, when watching the movie, I felt like a lot of the stuff that was going on was kind of disconnected, or... loosely connected. Not all of it made perfect sense to me, but in the end, it all kind of tied together and made a bit more sense, I guess. Though it requires you to just accept various concepts which may be based in different works of fiction as well as in Catholicism (not that I know much about that, but I'm sure some of the story elements diverged from what Catholics or any Christians believe). One thing which I am unable to accept in real life is the idea that suicide is a sin which earns you an unavoidable place in Hell, but I can accept that for the purposes of the story. Not sure what else to say except that the movie's quite dark, and disturbing, though fairly interesting, and there's also some humor, and the end... well the climax is pretty good, I'm not sure about the actual end. Not a bad movie, though it took awhile for me to get into it. And I probably don't need to ever see it again.
Oh, and there's a bonus scene after the end credits, which is nice, though I'm not sure it makes much sense (it suggests something that surely goes against the Christian concept of angels, though it's in keeping with a somewhat popular secular belief about them, I guess). Just thought I'd mention it, cuz you know, a lotta people don't bother checking to see if there are any bonus scenes. So now you know.
Nearly a decade after this movie was released, a new TV series called Constantine debuted, which is also based on Hellblazer, though it doesn't seem to be part of the same continuity as the movie.