The Devil's son Mammon tries to upstage the old man and create hell on earth--starting in Los Angeles, which is already pretty close. Mammon's secret helper is Gabriel the Angel who thinks humans ought to do more to earn God's grace. John Constantine (Keanu Reeves)--who chain smokes and like Santa Claus, can see who's naughty and nice (he unsuccessfully tried to commit suicide because of this 'gift') ends up saving Angela (Rachel Weisz) and committing suicide a second time so he can give the Devil a heads-up about Mammon's plan, which the Devil stops. This earns Constantine a favor; he talks the Devil into releasing Angela's twin sister Isabel (who jumped off a building) from hell - where suicides go - so she can enter Heaven. Because Constantine sacrificed himself for Isabel's salvation, God lets him into Heaven. But John Constantine just can't resist flipping the Devil the 'bird' one last time as he ascends, angering him to the extent that the Devil rips Constantine back from Heaven, cures him of terminal lung cancer (and slit wrists) and sends him back here on earth to fight evil another day.
Thanks, Barking Cats!