John Constantine (Keanu Reeves) was born with a gift or a curse to see demons and angels walking this earth disguised as humans. Demons and angels cannot make a person do anything, but they can whisper in your ear and give you an urge to do it. As a child, John could not understand what he was seeing, and he had enough of this life and wanted to leave this cruel world by killing himself. Unfortunately for John, he is saved from death, and now must atone for his act. John is given extraordinary powers, and becomes a guardian on earth. Whenever a demon breaks the rules, John exorcises them and sends them back to Hell. John is known by the police for working in the supernatural field of the occult, and Los Angeles police detective Angela Dodson (Rachel Weisz) wants his help in her investigation. Angela has discovered that spirits are walking the earth, and she believes they had some connection to the suicide of her twin sister. Angela becomes aware that John can see these spirits, and she wants to see them too. John reluctantly grants her wish. John and Angela must deal with representatives of both Heaven and Hell to save humanity. Douglas Young (the-movie-guy)
Set in Los Angeles, California, John Constantine (Keanu Reeves), a chain-smoking cynic, has the ability to see half-angels and half-demons true forms. He seeks salvation from eternal damnation to hell for a suicide attempt in his youth, by sending half-demons back to Hell with his sidekick Chas (Shia LaBeouf). However, full demons have begun attempting to cross over. Disturbed by this information and with his death looming over him due to lung cancer, Constantine seeks an audience with the androgynous half-breed angel Gabriel (Tilda Swinton). Gabriel tells him that as he does his exorcisms to escape Hell and for his own benefit, they are vain acts that wont spare him. Departing, he encounters LAPD officer Angela Dodson (Rachel Weisz), investigating the death of her twin sister Isabel, who leapt from the roof of the mental hospital she was committed to. Constantine is attacked by a true demon, but kills it. Upon attempting to speak with former witch-doctor turned owner of a nightclub that serves as neutral ground for half-breeds; Papa Midnite (Djimon Honsou) about this attack, he is rebuffed and dismissed. Constantine, with the help of his associates Beeman (Max Baker), Hennessy (Pruitt Taylor Vince), and Chas, begins investigating the situation.
Constantine tells Angela that God and Satan have a standing wager for the souls of all mankind. The rule for this wager is that angels and demons cannot manifest on Earth (however, half-breeds can), but can possess and influence humans. Afterwards, Constantine finds out Isabel was psychic, and her visions lead to her institutionalization. With the help of Hennessy and Beeman, Constantine finds out that the demons are working for Mammon, Satans son, who wishes to create his own kingdom on Earth. In order for Mammon to come to Earth, he needs divine assistance and a powerful psychic, for which reason half-demon Balthazar (Gavin Rossdale) locates Isabel and Angela. Angela reveals that she possessed the same gift but denied hers to the point that it became inactive. At her request, Constantine reawakens it so she can see demons and angels true forms.
Constantine attacks and interrogates Balthazar, who reveals that Mammon has obtained the Spear of Destiny, which has the blood of Jesus Christ encrusted on it. Angela is kidnapped to be used as the portal for Mammons entrance to Earth. Constantine storms Midnites club and after a brief scuffle, Midnite allows Constantine access to the chair, an old electric chair from Sing Sing Prison that had killed over 200 people. It grants Constantine visions which show him that the Spear was discovered in Mexico and that it is already in L.A. Constantine and Chas go to Isabels hospital, where Mammon is conducting the ritual to rise in Angelas body. After exorcising numerous half-breed demons, the pair interrupt the ritual and attempt to exorcise Mammon from Angela, costing Chas his life at the hands of an invisible party.
Using sigils on his arms, John forces the party, Gabriel, to reveal itself, but Constantine is promptly subdued. Gabriel resents Gods favoritism towards humanity, and the forgiveness they are readily given. Gabriel believes that by bringing Hell to Earth, humans can become worthy of Gods love through repentance and faith. Gabriel throws Constantine from the room and begins to release Mammon. As Gabriel goes to stab Angela with the Spear and release Mammon, Constantine slits his wrists. Time stops and Lucifer (Peter Stormare), who hates Constantine enough to collect his soul personally, arrives. Constantine tells Lucifer about Mammons plan and Lucifer sends Mammon back to Hell to keep Mammon from conquering Earth before him. Gabriel attempts to smite Lucifer, but is relieved of its holy powers and defeated.
In return for helping Lucifer, Constantine is owed a favour, but rather than asking for an 'extension' and be restored to life, he instead asks that Isabel be allowed to go to Heaven. Lucifer happily obliges, but, as he begins to drag Constantine to Hell, Heaven appears before him; by sacrificing himself for Isabel when there was clearly nothing in it for him, Constantine has proven himself worthy to enter Heaven. Infuriated, Lucifer heals Constantines wounds and cures him of his cancer so that Constantine would have a second chance to 'prove that [he] truly belongs in Hell'. Constantine retrieves the Spear and departs with Angela, but not before a now-human Gabriel attempts to goad Constantine into committing murder, only for Constantine to simply respond to his efforts by punching him to introduce Gabriel to pain.
Constantine then gives the Spear to Angela and instructs her to hide it where no one can find it. The final shot before the credits reveals that Constantine has switched from cigarettes to gum.
After the credits, Constantine visits Chas grave and sees Chas rise into Heaven as an angel.