Plot

Detective Angela approaches Constantine, a demon hunter, to help her investigate her twin sister's mysterious death. As he digs deeper, he realizes that demons are trying to enter the human world.

John Constantine is approached by Det. Angela Dodson who needs his help to prove that her twin sister Isabel's death was not a suicide. The dead woman was a devout Catholic and Angela refuses to accept she would have taken her own life. She's asked Constantine for help because he has a reputation for dealing with the mystical. In fact, he is a demon hunter whose sole purpose on Earth is to send demons back to the nether regions. John himself has been to Hell and knows that he is destined to return there on his death - but hopes his good deeds may find him a place in Heaven. As he looks into Isabel's death, he realizes demons are trying to break through to the human world, and his battles lead him into a direct conflict with Satan.—garykmcd

To make amends with God and earn his place in Heaven, taciturn occult detective and humankind's reluctant hero, John Constantine, finds himself caught in an interminable battle between the forces of good and evil. However, banishing rogue demons to the pits of Hell hasn't quite cut it, and when skeptical LAPD officer Angela Dodson asks John to shed light on the baffling death of her devout twin sister Isabel, the gifted supernatural investigator will finally get the chance to atone himself. As their fates become inextricably interwoven, the unlikely duo must join forces to unearth a sinister conspiracy and stop the impending Apocalypse. Is Constantine destined to save the world from evil?—Nick Riganas

John Constantine (Reeves) has literally been to Hell and back. When he teams up with a policewoman to solve the mysterious suicide of her twin sister, their investigation takes them through the world of demons and angels that exists beneath the landscape of contemporary Los Angeles.

The story of irreverent supernatural detective John Constantine, who has literally been to Hell and back. When Constantine teams up with skeptical policewoman Angela Dodson to solve her twin sister's mysterious suicide, their investigation takes them through the world of demons and angels that exists just beneath the landscape of contemporary Los Angeles. Caught in a catastrophic series of otherworldly events, the two become inextricably involved and seek to find their own peace at any cost.—unit publicist/WB

In the Mexican countryside, a scavenger named Manuel (Jesse Ramirez) finds a spearhead wrapped in a Nazi flag at the ruins of an old church. The spearhead is later revealed as the Spear of Destiny. Manuel becomes possessed and travels to the United States.

In Los Angeles, occult detective John Constantine (Keanu Reeves) (a chain-smoking cynic with the ability to perceive the true visage of half-angels and half-demons on the human plane. The rule for this wager is that angels and demons cannot manifest on Earth (however, half-breeds can), but can possess and influence humans.

John is damned to Hell for attempting suicide-a mortal sin-and has terminal lung cancer) is called in by Father Hennessy to exorcise a girl possessed by a demon trying to break through to Earth, which should not be possible under the rules of a standing wager between God and Lucifer for mankind's souls. Constantine's driver and apprentice Chas Kramer (Shia LaBeouf) waits in the car as Constantine doesn't believe he's ready to exorcise demons.

Constantine is disturbed that full demons are trying to cross over and with his death looming over him due to lung cancer, he meets with the androgynous half-angel being Gabriel (Tilda Swinton). He asks Gabriel for a reprieve from his impending death due to the cancer. Gabriel declines, telling Constantine that he exorcises demons for selfish reasons and cannot buy his way into Heaven, because of the life he took.

After being assaulted by another demon (this time in the form of insects), Constantine goes to Papa Midnite (Djimon Hounsou), a reputed witch doctor who runs a club serving as neutral ground where half-breeds do not have to conceal themselves. Papa tells Constantine that he is the one soul that Lucifer would come himself to collect. Midnite doesn't believe Constantine's claim of demons crossing over. Constantine leaves after exchanging hostile words with the half-demon Balthazar.

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.

Elsewhere, a woman named Isabel Dodson commits suicide in a psychiatric hospital. Her twin sister, Detective Angela Dodson (Rachel Weisz), refuses to believe that Isabel, a devout Catholic, would kill herself. Watching security footage of Isabel's suicide, Angela hears her say Constantine's name. Angela finds Constantine and asks him to help investigate her death. After they are attacked by winged demons, which Constantine believes were targeting Angela, he agrees to help.

Constantine briefly transports himself to Hell through the possession of a familiar and sees Isabel damned to eternally relive her suicide. As proof, he is able to bring back her hospital wristband. He finds out for the first time that Isabel & Angela are twins. Constantine explains to Angela that he can see the true nature of the half-breeds. He committed suicide to escape his visions as a teenager and his soul was sent to Hell, but he was revived by paramedics two minutes later; for the sin of taking his own life, his soul is still condemned to go to Hell once he dies. John was given extraordinary powers, and became a guardian on earth. Whenever a demon breaks the rules, John exorcises them and sends them back to Hell.

Elsewhere, Father Hennessy (Pruitt Taylor Vince) investigates Isabel's corpse and finds a mysterious mark on her wrist. Balthazar (Gavin Rossdale) drowns him in alcohol before he can inform Constantine. Constantine and Angela search Hennessy's body at the crime scene and find the symbol burned into Hennessy's palm.

The two examine Isabel's room in the hospital and find a clue (left on the window only revealed by Angela breathing on it) pointing to a prophecy in the Satanic Bible and enlist the help of Constantine's weapons supplier, Beeman (Max Baker), who informs them that the mark belongs to Lucifer's (Peter Stormare) son, Mammon. The prophecy states that Mammon will attempt to claim Earth as his own kingdom. In order to cross over, Mammon requires both a powerful psychic and assistance from God. Constantine and Angela find Beeman's corpse engulfed beneath a swarm of flies.

Angela tells Constantine that Isabel, a clairvoyant and a psychic, was committed by their parents. Angela had the same gift but suppressed it, and now feels guilty that she didn't corroborate Isabel's visions, having feared the treatments Isabel was made to go through. At Angela's insistence, Constantine reawakens her psychic ability through a near-death experience. She immediately finds a clue pinpointing Balthazar as an accomplice to the plot; Constantine interrogates Balthazar, who reveals that Mammon has the Spear, stained with the blood of Christ-the assistance from God. Angela, now the psychic in place of Isabel, is abducted and possessed by an invisible entity.

Constantine convinces Midnite that the demons are breaking the rules. With Midnite's help, Constantine finds out how the Spear emerged and Angela's location. Constantine arms himself and goes to the hospital, reluctantly bringing Chas along. The two fight their way through an army of half-demons to exorcise Angela. Believing they have won, Constantine witnesses an invisible entity kill Chas, which turns out to be Gabriel. Resenting God's favoritism towards humans, Gabriel plans to unleash Hell on Earth to weed out those deemed "unworthy" of God's love. Gabriel casts Constantine from the room and prepares to use the Spear to cut Mammon free from Angela.

Out of options, Constantine slits his wrists. As he bleeds out, Lucifer arrives to personally collect his soul. Constantine tells Lucifer of Mammon's plan to usurp him and Gabriel's part in the plan. Confronted by Lucifer, Gabriel threatens to 'smite' him in God's honor; however, the attack against Lucifer comes up short, revealing to both Gabriel and Lucifer that Gabriel no longer has divine protection. Lucifer proceeds to burn Gabriel's wings, making Gabriel mortal. Lucifer banishes Mammon back to Hell and grants Constantine a wish out of gratitude.

But rather than asking for an 'extension' and be restored to life, Constantine asks that Isabel be released to Heaven. Lucifer complies but realizes too late that he cannot take Constantine to Hell as by selflessly sacrificing himself, Constantine is granted entry to Heaven. Constantine begins to float upwards, but Lucifer, infuriated, intervenes before Constantine can be fully taken by healing Constantine's injuries and cures him of his lung cancer by pulling out the tumors in his lungs, hoping he will eventually damn himself again.

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.

Sometime later, Constantine, now making an effort to quit smoking (Constantine has switched from cigarettes to gum), entrusts the Spear to Angela and instructs her to hide it where no one can find it. Finally, he visits Chas's grave, where Chas appears in an angelic form and ascends into the night sky.