Plot

When a debt puts a young man's life in danger, he turns to putting a hit out on his evil mother in order to collect the insurance.

Finding himself in considerable debt, Chris, a Texan drug dealer, decides the only solution is to murder his mother to collect the insurance money. Getting together with his father, the ex-husband of Chris' mother, they decide to hire Joe Cooper (a contract killer) who also happens to be a police detective. The plan is that the money will go to Chris' sister Dottie. However due to the size of the contract fee, Chris agrees that Joe can take Dottie as a retainer until the insurance comes through.

Chris Smith (played by Emile Hirsch) is in a bind. He owes a loan-shark a fair bit of money, and the man has threatened to kill him if he does not produce the money within a few days. He does not have the money. He strikes upon a get-rich-quick plan: have his estranged mother killed and collect on the insurance money. He involves his father, Ansel (Thomas Haden Church), who buys into the deal for a share of the profit (him and Chris' mother are divorced and don't see eye-to-eye, so it wasn't a tough sell). Ansel then involves his 2nd wife, Sharla (Gina Gershon) and, due to her finding out about the plan, his daughter, Chris' sister, Dottie (Juno Temple). They hire a professional killer (who happens to also be a cop), "Killer Joe" Cooper (Matthew McConaughey). When they cannot pay him upfront, they reach an unusual deal on how to retain his services. This complicates matters, a lot.—grantss

In West Dallas, 22-year-old drug dealer Chris Smith comes to the trailer park home of his dimwitted father Ansel and his cheating stepmother Sharla after his mother and Ansel's ex-wife, Adele, has thrown Chris out of her house. To make matters worse, Adele stole the cocaine that Chris was supposed to sell, and he is now desperate to pay the $6,000 debt he owes his supplier Digger Soames (Marc Macaulay). Adele used the money from the cocaine to fix her gold Cadillac.

Chris (Emile Hirsch) finds himself with loan sharks threatening to kill him. Chris decides the only solution is to murder his birth mother, Adele to collect the $50,000 of insurance money. He has been told by his mother's boyfriend Rex that the sole beneficiary will be Chris' younger sister Dottie (Juno Temple) who lives with Ansel and Sharla. Ansel, Sharla, and Chris live in a mobile trailer, and are a completely dysfunctional unit. The trio cannot stand each other, and Dottie stays in her own room, in her own section of solitude, hoping for peace and a better life.

Assuming Dottie would share any money, she gets with them, Chris tries to rope his father, Ansel (Thomas Haden Church), into a conspiracy to kill Adele - who is Ansel's ex-wife - to get the money. Chris tells his dad that he has heard of a guy who can help them - Joe Cooper (Matthew McConaughey), a police detective who has a side career as a contract killer. Chris doesn't say who told him about Joe. The idea was to keep the whole thing secret from Dottie as they were not sure about how she would react. Ansel eventually agrees, planning to split the money four ways between themselves, Dottie, and Ansel's new wife Sharla (Gina Gersho). Sharla works as a waitress at a pizza place. Dottie hears the plan as they are talking and agrees that it's a good idea.

Chris sets up a meeting with Joe and Joe arrives at the trailer home. Only Dottie was at home and Joe gets to spend some time with her. Dottie finds that Joe is a detective, and she knows why Joe is at the trailer home. Chris calls and says that he and Ansel were unable to get away from work. He asks Joe to meet them at a muffler shop where Ansel works. Through Dottie's interaction with Joe, it is revealed that Adele (her mother) tried to kill her once when she was an infant. Chris had asked Joe to bring Dottie along.

Since Chris and Ansel are broke, the plan almost falls apart when Joe demands $25,000 in advance. Joe was about to walk away and sees Dottie dancing outside. Joe is interested in Dottie, who is implied to be developmentally disabled from the murder attempt by Adele and offers to take her as a "retainer" until the insurance money comes through. Sharla is worried as Dottie has never been on a date before and unless Dottie is prepared, Sharla believes that she will end up disappointing Joe and the whole thing will fall apart. Sharla gives Dottie money to buy a new dress.

Joe "dates" Dottie and then appears to be staying over at their home and having sex with her regularly. Digger has his biker henchmen beat Chris and threatens to have him killed unless he pays what he owes. Having a change of heart, Chris asks Joe to call off the hit and stop seeing Dottie, only to discover that he has already killed Adele. With the help of a reluctant Chris, Joe leaves the body in a car and sets it on fire.

After Adele's death is discovered, the family learns that the insurance claim actually pays to Adele's boyfriend Rex, not to Dottie. Ansel and Sharla confront Chris about this. Chris then admits he originally heard the details about the policy from Rex, who also told him about Joe, thereby duping Chris into hiring Joe to kill Adele. Ansel blames Chris for the fiasco and tells him he should just kill himself. Chris tries to persuade Dottie to run away with him to escape Digger and Joe, both of whom expect to be paid. Dottie agrees to go with Chris if she can see Joe first.

Ansel and Sharla go back to their home after the funeral. Joe is already there with Dottie. He comes out of her room and begins asking seemingly casual questions of Sharla. They become more and more pointed until ultimately, they force her to admit that she knew the policy was really $100,000 (since accidental death doubled the payout). Joe also has retrieved some nude photos of Rex which Sharla had taken which Joe uses to prove to Ansel that she has been having an affair with Rex. Joe also shows them a check he has obviously taken from Rex, payable to Rex for $100,000. Angered, Ansel declines to protect Sharla when Joe punches her and forces her to simulate oral sex on a piece of fried chicken.

Joe knows Chris is coming to take Dottie away and he threatens to murder Ansel and Sharla if they don't help him prevent Chris from taking her. Joe tells Sharla to put dinner on the table. After Chris arrives and they are all seated, Joe announces that he and Dottie will be married.

Chris refuses to let them, ordering Dottie to leave with him, as Joe tells her to stay where she is. For a moment she sits there. Then she gets up and turns, while the men yell out at her. When Chris threatens Joe with his gun, Sharla stabs Chris with a steak knife and Joe tackles him. Ansel and Sharla rush to assist Joe as he beats Chris, not wanting to be killed if Chris flees with Dottie. Dottie recovers the gun and, in a rage, fires several shots, killing Chris and seriously wounding Ansel. Dottie turns the gun on Joe, telling him that she is pregnant. Joe appears overjoyed as he inches closer to Dottie.

The film ends just as Dottie moves her finger back on the trigger.