God and Lucifer are engaged in a war for the souls of humanity; a standing wager for the souls of all mankind. Angels and demons are forbidden to manifest on Earth, but they are allowed to possess and influence humans, and half-breeds are used to peddle influence. Exorcist John Constantine (Keanu Reeves) was born with the power to see angels and demons. At age 15, he committed suicide to escape his visions, but he was revived after spending two minutes in Hell, though John explains that since time moves differently in Hell 2 minutes feels like a full lifetime. Consequently, his soul is bound for Hell when he dies for the sin of taking his own life.
John exorcises a girl possessed by a soldier demon trying to break through to Earth, something that should not be possible, as demons cannot take their true physical form on the mortal plane. John seeks an audience with the androgynous half-breed angel Gabriel (Tilda Swinton). He asks Gabriel for a reprieve from his impending death from lung cancer; but Gabriel declines, telling John that his motives for exorcising demons are selfish and will not earn him entry into Heaven. After he leaves Gabriel, John repels an attack by a full demon out in the open. This encounter prompts him to meet with former witch doctor Papa Midnite (Djimon Hounsou) who informs him that all of Hell is waiting for him to die and that he is the one soul Lucifer would come to collect himself. There, John also encounters half-breed demon Balthazar (Gavin Rossdale). Midnite refuses to become involved, wanting to retain the balance between Heaven and Hell. John begins investigating the situation with his associates Beeman (Max Baker), Hennessy (Pruitt Taylor Vince), and Chas Kramer (Shia LaBeouf). Detective Angela Dodson (Rachel Weisz) shows up at Constantine's apartment to ask for his help investigating Isabel (Rachel Weisz), her identical twin's, death. Isabel leapt from the top of a psychiatric hospital, where she was a patient; and, despite camera footage showing it, Angela is convinced that Isabel would never commit suicide. At first, John mocks her and denies her request for help; but, after demons chase after Angela on the street outside, Constantine agrees to help.
Later on, John wants to see if Isabel is truly in Hell, he then takes a hold of Angela's cat as a means of teleportation and briefly transports himself into the depths of hell where he finds Isabel, whose soul is damned to eternally reliving the moment of her death. Hennessy's and Beeman's research leads them to conclude that Lucifer's son, Mammon, is plotting to break through to Earth and claim it as his own kingdom. To do so, Mammon requires a powerful psychic and assistance from God. Balthazar kills Hennessy and Beeman, and Angela reveals that she and her sister possessed the same gift as John. Angela rejected her visions and they eventually stopped, but Isabel embraced them and was institutionalized for it. John reawakens Angela's psychic ability through a near death experience, then hunts down and interrogates Balthazar who reveals that Mammon has obtained the Spear of Destiny, which is stained with the blood of Jesus Christ. Afterwards, John proceeds to kill Balthazar and he and Angela then leave. Angela, now possessing the psychic abilities Mammon requires, is abducted by an unseen force and brought to the hospital where Isabel supposedly jumped to her death. John convinces Midnite that the balance is no longer in force, and asks to use "The Chair"; an old electric chair from Sing Sing. The Chair shows John a vision of how the spear was discovered in Mexico and has been brought to Los Angeles. John and Chas head to the hospital and interrupt the ritual, but Chas is beaten to death by an unseen force in the process.
Using incantations and sigils tattooed on his arms, John forces the unseen force, revealed to be an invisible Gabriel, to reveal itself. Gabriel subdues John, admits responsibility for the plan to release Mammon, and reveals the details. Gabriel laments Godâs favoritism towards humans and believes that bringing Hell to Earth will enable those who survive to become truly worthy of Godâs love through suffering, repentance and faith. Gabriel then casts John from the room. As Gabriel moves to stab Angela with the Spear and release Mammon, John slits his wrists and dies. Time stops as Lucifer arrives to personally collect his soul and John tells him about Mammonâs plan to usurp him. Gabriel attempts to smite Lucifer but cannot as God has taken away Gabriel's power, allowing Lucifer to burn Gabriel's wings. Lucifer sends Mammon back to Hell. In return for his help, Lucifer grants John a favor; instead of a longer life, he asks Lucifer to allow Isabel to go to Heaven. Lucifer agrees, but then finds that he is unable to drag John to Hell; John's noble sacrifice having granted him entry to Heaven. Infuriated at losing John Constantine's soul, Lucifer painfully resurrects him and removes his cancer, claiming that John will eventually prove he belongs in Hell. Freed from Mammon's possession, Angela departs with John, leaving behind the now human Gabriel. Some time later, John gifts the Spear to Angela, asking her to hide it somewhere not even he can find it. As she leaves, instead of producing a cigarette, he starts to chew on some nicotine gum.
In a post-credits scene, John visits Chas' grave. Chas appears before him as an angel and flies upward to the sky.

Question: Why does John seek Gabriel's help?
Answer: To get a reprieve from his lung cancer. 
Alternative answer: To save him from lunch cancer