From the dawn of humanity, prisons have served the purpose of capturing and containing wicked individuals. Even if these malevolent men manage to break free, they are ultimately brought to justice. John Milton (Nicolas Cage) has made an audacious escape from Hell. The torment he endures there is bearable, yet he finds it utterly unbearable to watch the footage of his daughter’s murder. He manages to steal the formidable Godkiller gun from Satan himself to pursue Jonah King (played by Billy Burke), a sinister cult leader responsible for his daughter's death and her husband's murder. King plans to use Milton's infant granddaughter in a ritual sacrifice, believing it will unleash Hell upon the Earth.
Milton's daughter once faced personal turmoil during her college years, where she met Jonah, who lured her into his dark world of Satanic magic. After realizing the truth, she fled, having stabbed Jonah in the face, and sought refuge in marriage. Following a trail of destruction through Laughter, Colorado, where Milton interrogates and dispatches a few of King's followers, he learns that the ritual is set to occur at Stillwater, a derelict prison in Deacon's Tree, Texas.
On his journey, he visits a diner where he crosses paths with Piper (Amber Heard), a waitress who confides in her coworker Norma Jean (Katy Mixon) about her decision to withhold intimacy from her boyfriend, Frank (Todd Farmer), until he proposes. During his stop, Milton discovers that the actual Stillwater is located in Louisiana, and, to his dismay, that the full moon, which is crucial to the ritual, is imminent.
Piper's situation escalates when she quits her job after a disturbing encounter with the diner owner. Milton, looking for transportation after his car fails him, sabotages Piper's vintage 1969 Dodge Charger, claiming he can fix it later if she agrees to give him a ride. Meanwhile, Piper confronts her cheating boyfriend Frank, leading to an intense altercation where Milton intervenes. He seizes Frank's car, whisking Piper away to continue his quest for vengeance.
However, the supernatural enforcer known as The Accountant (William Fichtner) is dispatched by Satan to retrieve Milton and reclaim the Godkiller. After learning Milton's whereabouts from Norma, The Accountant ruthlessly eliminates Frank and manipulates state troopers to aid his mission.
During their escape, Milton and Piper stop at a bar where she retires to her room while he momentarily gets involved with waitress Candy. In a shocking turn, King and his goons attack Milton at a motel, but the fury that prevails within him leads him to dispatch them before King manages to flee. As they pursue King, Milton explains to Piper that retrieving his granddaughter is vital as she represents the last connection to his deceased daughter.
Delving deeper into the chaotic chase, Milton uses the Godkiller to thwart The Accountant, who crashes his vehicle. Their search leads them to a church filled with King's followers where they unexpectedly face capture. In a shocking confrontation, King boasts about Milton abandoning his daughter, and as chaos ensues, he shoots Milton, brandishing a femur bone belonging to Milton's daughter.
The danger intensifies as King kidnaps Piper, leaving Milton for dead. However, the resilient Milton revives with lethal intent, hunting down the RV containing Piper. After a suspenseful fight, with Piper managing to escape at a critical moment, Milton enlists the help of an old ally, Webster (David Morse), who provides them with a 1971 red Chevrolet Chevelle SS. Here, Piper learns of Milton's Undead status and his past, which involved a gang that ultimately allowed King to manipulate his daughter into his cult.
Despite the adversities they face, Milton gives orders for Piper and Webster to leave, but she stands firm, deciding to fight alongside him for the first time for a justified cause. As they evade Law enforcement with the Accountant's unexpected assistance, they arrive at Stillwater, where a final confrontation with King is inevitable. The Accountant captures Piper and demands the Godkiller in exchange for a moment with Milton, but he allows Milton to rescue his granddaughter first.
With a brutal clash underway, Milton fights through King's followers, desperate to save the innocent child. In her courageous act, Piper seizes the Godkiller just as King gains dominance over Milton. A struggle ensues, but with sharp precision, Milton claims victory by finally using the Godkiller against King. As Milton cradles his granddaughter, the Accountant allows him a moment of emotional farewell before he is gravely wounded.
In a poignant farewell, Milton "dies" in front of Piper and Webster, but it is soon revealed that he is not fully finished yet. Embracing his life's promise to Webster, he partakes in a drink from the remnants of King's skull, sealing his fate as he prepares to return to Hell, threatening to escape again if the punishment is too severe. The Accountant expresses anticipation for their meeting, as they drive a 1957 Chevrolet Bel Air right into the mouth of Hell.
