We open up with a voice over by The Accountant (William Fichtner) as we pan over an aerial view of hell. He says that Hell is nothing more than a bunch of wannabe "Bad ass Motherfuckers" caged up wanting to be set free in the world. Every once in a while, he continues, one does gets out. It is his job to reclaim the soul back to hell, so the accounts can be "balanced."
We then cut to three men driving haphazardly in a beat up pickup truck. They are scared and keep looking over their shoulders for "him." They are afraid that he will catch them for what they had done to his daughter. They finally see the muscle car containing John Milton (Nicolas Cage) right behind them. They freak out more and resort to more drastic evasive maneuvers. When it looks like they finally lost him, they find him in front of them. Milton shoots the car with his shotgun, which causes the truck to hit his and flip over in an over the top manner.
Two of the men try to charge Milton. One has a crowbar in his hand, ready to strike. Milton shoots his arm off, and the man falls to the ground from blood loss and shock. The other tries to shoot him, but Milton catches him in the leg and hobbles him. Milton goes to the man in the car. The man is defiant to the last second saying, "I'm not going to tell you anything about that baby. Nothing." Milton replies, "I know you won't." and shoots him in the head, splattering the inside of the truck with gore. Going to the last survivor and threatening him with death, the man cracks. He tells Milton the baby is in Smithfalls, Texas, but there is nothing he can do. The child will die and "hell will walk the earth." Milton shoots a puddle of gas with the shotgun, making the truck explode. "HELL is already walking the earth." he replies.
We know find ourselves at a greasy-spoon diner called Lou's. Two waitresses Piper (Amber Heard) and Norma Jean (Katy Mixon) are chatting up as they work. Piper is telling Norma Jean that she is withholding sex from her boyfriend, so he will stop messing around and marry her. He didn't buy her a ring (he's unemployed), but he has made an offer of marriage. The girls scream in happiness, but their boss Fat Lou (Jack McGee) tells them to get back to work. Piper tends to a family that cannot afford much food and gives them free muffins on the house. Norma Jean meanwhile tends to Milton who is looking at some maps.
Norma Jean begins to flirt heavily with Milton. He orders a black coffee with sugar. When she returns, Milton doesn't say much and asks about Smithfalls. Norma Jean replies it is actually a prison in Louisiana. Milton notices a muscle car and asks if it is hers. Norma Jean says no it is Piper's but if he is "looking for a ride" (i.e. sex) she could oblige him. They begin to make out before Milton stops her by grabbing her at the throat saying "You forgot the sugar." Norma Jean leaves bewildered, and Milton pours sugar in his coffee. Meanwhile, Lou sexually harasses Piper after she gives the free food away. She responds by grabbing his nuts and shoving money in his mouth. She quits over his abuse and runs to her car. Milton is long gone from the diner.
Piper races off in her car but it cuts out suddenly. Forced off the road, she begins to look before Milton shows up and offers to fix it for a ride (he tampered with it on purpose). She offers him one as far as where she lives. They fix the car and drive off. Unknown to them, The Accountant has appeared from hell on an abandoned road and begun to walk.
The Accountant finds his way to Lou's and asks where Milton had gone. Norma Jean points him in the right direction.
At Piper's neighborhood, Milton thanks her for the ride and goes on his way stopping at a phone booth. Piper goes in to find her boyfriend Frank (Todd Farmer) having sex with the local floozy Mona (Christa Campbell). Piper is understandably pissed off and drags the naked Mona outside for everyone to see with Frank not far behind making lame excuses. Mona is unrepentant and says she and Frank are in love but that just makes Piper more angry and she knocks her out cold. Piper is grabbing her things and Frank balks at her taking the car. Piper says it is not his choice she was the one paying for it. When he persists, she punches him hard twice, causing him to punch her in response. Milton sees all this. Frank helps Piper up, and she spits blood in his face. He hits her again. Milton beats Frank and causes an air conditioning unit to fall on him and knock him out.
Milton takes the car and takes Piper with him. While he is driving, he sees what happened to his daughter. She was put under the spell of cult leader Jonah King (Billy Burke). When she gave birth, King took her baby and forced her to give him oral sex. She refused and bit his penis clean off. Enraged, King cut her throat wide open.
Piper wakes up. Milton says she was going to Florida anyway, and he has business in Louisiana so can he go with her till then. She agrees.
Milton and Piper go to a bar/hotel where Milton knows the order Roy. Roy allows them to stay but tells them they have to be gone by dawn. They sit down where the bartender flirts with Milton. Piper leaves to get a shower and "do her nails", roping a busboy along with her. Piper tells Milton to go after the waitress as one never says at the end of their life, "God, I wish I hadn't fucked so much."
The Accountant goes to see Frank and question him about Milton. Frank is not in the mood to talk and tries to hurt the Accountant with a bat. The Accountant easily dodges the attack, breaks the bat in two halves and uses one to pin Frank to the wall until he gives him the information he wants. Satisfied, The Accountant leaves but not before taking the other part of the part and spearing it into Frank's face. Cops show up, and The Accountant impersonates an FBI agent and brings the two troopers to help him kill Milton.
Piper is in her room, getting her nails done by the busboy. The man asks if they are going to have sex. Piper says it depends on how well he does her nails. She can hear Milton and the waitress have sex very loudly next door, and it irritates her. She sees some men pull into the hotel, and it makes her suspicious. She goes into the bar and sees a report about Jonah King and how he killed a married couple and stole their child. She hides when she sees it is him and several of his men. They had followed them both with intentions to kill them. King tells his men to kill them both. Piper goes to the bar and sees garden tools as decor and is able to get a gardening hoe down off the wall.
The waitress, Candy (Charlotte Ross) and Milton are continuing to have sex. Candy asks why Milton isn't naked, and Milton replies "I never get naked before a gun battle." On that cue, King's men burst in and try to kill Milton. Milton kills them while still inside Candy. Milton at one point is stun gunned by a man, which shocks Candy in the process. Milton shoots him in the foot which leaves him open to a head shot. The final man has the drop on Milton, but Piper comes and smacks in the head with the hoe and kills him. She looks to her room and sees the busboy dead. Milton and Piper race out of the rooms. However, The Accountant and the deputies show up and begin to shoot at them. Milton returns fire while one deputy flanks them. He has a clear shot on Milton and Piper is forced to shoot him which causes him to fire his own gun at his partner, hitting him in the head. Milton and Piper escape to chase Jonah's van down while smirking at The Accountant.
Piper and Milton are close to Jonah King's van when The Accountant crashes into them with the squad car. He asks Milton what he is doing, and it is over. Milton is not willing to accept that and tells Piper to get the gun he had in his bag. Piper finds a box labeled "God Killer" and hands it to Milton. Milton fires a shot at the shocked Accountant. The Accountant barely dodges it, but it gives him a scar on his face and causes his car to careen off the bridge. Piper asks what kind of gun that was. Milton says its for things "like him." They move on while in a humorous scene, two stoners find the Accountant's car and he bursts out of, hitting one with a car door. When the kid says The Accountant almost killed him, he replies, "Barely. I won't see you again till you are 73." He looks to the second stoner and notes, "You I'll see in three months."
Piper sees Jonah's van at a church, and they pull over. Milton tells her to stay outside. Inside, he sees what seems to be a normal prayer group. However, he sees a small infant and realizes that is his grand daughter. He is quickly subdued however, (as is Piper). It was a trap laid by Jonah. He comes out and explains how his Milton's grandchild will help bring a new world order through sacrifice. Milton laughs at the cult leader trying to explain himself to him, as he was the one who murdered his daughter. Milton tells him something no else knew. The night Jonah killed his daughter, she bit his dick off in the last moment of defiance. Jonah swore his right hand man to secrecy. The man stutters at this, and Jonah shoots him in the head for thinking he told. Milton tells Jonah to give him the child. Jonah response is to shoot Milton in the head and kidnap Piper. Before he goes, Jonah tells Milton that his cane is actually made from his daughter's femur bone. Jonah places them all in a van and tells Piper that he will enjoy breaking her, just like he did Milton's daughter.
Meanwhile, Sheriff Cap (Tom Adkins) is investigating the hotel slayings and is also on the trail of Milton. He vows to kill them for the deaths of the officers.
Back at the church, a few left behind followers plan to burn and bury the bodies in the backyard. It is then that Milton wakes up and proceeds to kill several of the men and critically wounding one. He races off in Piper's car with several following in two cars.
Piper is resistant to capture be can do little as she tempts death by the cult members. When she realizes Milton is not too far behind, she begins to fight back, starting with a woman who threatened to kill her several times. Milton takes care of one of the cars that were chasing him, killing the driver in a warehouse. Piper throws the woman out the camper's back window, and she begins to fire on Milton who is right behind the van. The woman cult member is then run over by the second pursuing car. Milton swiftly takes out the second car and races to save Piper.
Jonah holds a gun to Piper saying she is not worth taming, and he is just going to kill her instead. Piper faces this and says "Well, between here and then I am going to FUCK you up." Jonah puts his gun down and begins to fight with Piper. He seems to get the upper hand only for Piper to stab him with his necklace (just like Milton's daughter did). Milton races up behind the car and Piper seeing her only shot, jumps out the window and grabs Milton's hand. Jonah shoots the car with his revolver, so they can no longer follow him. The car putters to a stop. Milton wants to continue on, but Pipers wants answers. Milton explains more or less, that he broke out of hell to save his grand daughter from being sacrificed to Jonah King's cult. Having no choice, they push the car to a friend of Milton's.
The Accountant finds the church where Milton previously was. One of the wounded men is still alive. The Accountant wants to know why Milton is out. The man explains Milton is coming after them because they have the child and intend to sacrifice it.
Milton and Piper are picked up by Webster (David Morse) who is incredulous that Milton stands there as Webster "carried his coffin." He tows there car back to his shop. There Webster explains Milton and him were in a bad crowd. Milton was a bad husband but a great father who eventually had to abandon his daughter to protect her. They go to see Milton who is watching a fire burn. Webster offers a beer but Milton declines. Milton says the fire in hell doesn't bother him, it is the "entertainment." The damned souls are forced to watch the suffering they caused in a loop for all eternity. So he gets to watch his daughter cry and curse him at his funeral, join the cult, and watch his son in law and daughter be butchered over and over. He tells Webster he will only have a beer when he drinks it out of Jonah King's skull. Webster can't fix the car but can give him another. They pick a red Chevelle.
Piper wants to go, but Webster tells Milton he is wary about letting more innocent blood be shed (meaning Piper). Milton tells Piper she can come, but he cannot guarantee her safety. Piper tells him that she has been waiting her whole life for a purpose and that she is with him till the end.
They get on the road but thanks to Jonah King following them and tipping off the cops, they are caught in a blockade. Sheriff Cap tells his men, "When I say aim for their tires, what I really mean is aim for their heads. Aim for their tires." Milton and Piper stop inches away from the blockade only to see The Accountant in a tanker containing hydrogen rolling their way. The Accountant breaks up the blockade giving the two a chance to escape. When the Sheriff demands an explanation, he procures his FBI agent badge again and makes them stand down. Sensing one of the men is a satanist, he orders him to call Jonah King and tell him Milton is dead.
Milton and Piper make it to Smithfalls prison. Milton takes a look and tells Piper to stay. He notices dozens of satanists, including the cop follower The Accountant met. When he returns he finds that The Accountant has taken Piper hostage. Here, we find out why he was after Milton; not only did he escape Hell but he stole the God Killer; A gun so powerful that anything shot with it goes neither to heaven or hell but fades to nothing. The Accountant says the devil will be mad about the theft but Milton replies, "What's he gonna do, not let me back in?"
The Accountant explains to Piper that Lucifer is actually a soft spoken entity; more like a prison warden who has to keep the animals at bay. What he REALLY hates is when a cult does something like sacrifice a child in his name. So now The Accountant would like the cult dealt with so they can be punished in hell. He tells Milton to leave the God Killer with him and save his grand daughter.
Jonah gets ready to sacrifice the child, but Milton drives down and begins to slaughter all his followers. The cop and another satanist see The Accountant and Piper in a watch tower and pursue them. They plan to kill them both, but The Accountant uses his special quarter to pierce their skulls and kill them dead.
Milton has killed almost all of Jonah's men when Jonah sets a trap that causes Milton's car to explode. Jonah goes in for the kill, but Piper wields the God Killer and tries to kill him. She misses and kills another follower instead, knocking her and the gun to the ground. Jonah orders his last subordinate, the woman who cared for the child to kill the baby at his command but it is clear she is unwilling. Jonah begins to taunt the downed Milton that he will destroy the last good thing in his life. He commands the woman to kill the child but she will not. Meanwhile, Milton sees the God Killer within his grasp. The Accountant momentarily distracts Jonah giving Milton the chance to fire the gun. It hits Jonah dead center, disintegrating his body to nothing, save for part of his skull.
The Accountant retrieves the child and asks the woman if she would have done it. Her lack of response confirms his suspicions; she wouldn't have. She wasn't that evil. He waves her off, saying "I'll see you soon."
Milton gets to hold his grand daughter for a moment and hands it to Piper. He tells Piper to take care of her, to love the child as if it were her own. Piper is anguished at Milton dying but promises to protect the child. Webster drives up, and Milton says he will look over Piper and the child as long as he can. Milton dies, and Piper breaks down. She goes to Webster, and when they look back Milton and The Accountant are gone.
They're not actually, just used a parlor trick to make it seem like they did. Milton drinks beer from Jonah's skull (like he promised he would) and The Accountant procures their ride back to hell. Milton tells him he will go back, but he does not intend on staying. The Accountant says he can't wait for it, for it should be fun. Milton asks for the keys and they drive back into the gateways of hell. Milton his mission complete, smiles.