Lou (Kristen Stewart) works at a hard-core gym, unclogging toilets and being pressured by Daisy (Anna Baryshnikov) for a date.Jackie (Katy O'Brian) arrives in town and has sex with JJ (Dave Franco) in order to secure a job at a gun range. She has nowhere to stay, so she sleeps and works out underneath a nearby bridge.Lou visits her sister, Beth (Jena Malone), whose house is filled with children and suffers clear signs of domestic abuse from her husband, JJ. He leaves for work where he shows Jackie around and introduces her to the owner (Ed Harris).Jackie comes to work out at the gym, and Lou is immediately smitten by her. Her fantasies are interrupted by two FBI agents asking about her father, who she says she hasn't talked to in years. Lou and Jackie chat outside, but are interrupted by two patrons asking Jackie out on a date. She refuses, then punches one in the face after he attempts to grab her. He hits her back and Lou rushes her inside. They talk about Jackie's dream to go to Vegas to compete in the finals for a bodybuilding competition, so Lou offers her a stash of steroids from her office. While Jackie is initially reluctant, she agrees and Lou injects her with a vial. The two are clearly attracted to each other and begin making out, then leave for Lou's apartment to have sex.The following morning, Lou laments that Jackie's working at the gun range, as her estranged dad, Lou Sr., owns it. Lou agrees to let Jackie stay with her, leading to a period of love, sex, and steroid injections. Back at the gun range, Lou Sr. runs a criminal operation smuggling guns to Mexico and teaches Jackie how to shoot a gun.Lou and Jackie go on an awkward double date with JJ and Beth, who has another set of bruises and cuts. Lou follows JJ to the bathroom to threaten him, but he reveals that he slept with Jackie and suggests that she's probably just using Lou for the gym and housing. They fight on the way home, but speed away together after Lou spots the FBI agents.The following day, Lou gets a call that Beth is in the hospital with severe face and head injuries. Lou is furious, but the police will not arrest JJ unless Beth agrees to press charges, which she always refuses to do. Upon seeing Lou sobbing, Jackie's muscles begin to swell and she flies into a rage and drives to Beth's house to violently murder JJ. Lou spots her truck in front of Beth's house and enters to find JJ's body and a catatonic Jackie. After being initially horrified, Lou sets to work cleaning up the mess and puts JJ in the back of his own car. On the way to dispose of the body, Daisy spots Lou and knocks on the window of JJ's car to chat. The girls push the car into a large gorge, then set it on fire, sending smoke billowing into the sky. Lou assures Jackie that there are other things in the gorge that will point the police to someone else, so all they need to do is stay put in the apt. While Lou cleans up Beth's house, Jackie takes many vials of steroids and breaks out of the house to go work out, which infuriates Lou. Still hyped up on steroids, Jackie headbutts Lou and hitchhikes to Vegas for the bodybuilding competition. While her individual performance originally goes well, she has a panic attack and vomits on the stage. When she sees other competitors laughing at her, she viciously attacks them and is arrested.Lou Sr. confronts Lou at the hospital about leading police to the gorge, as it has attracted attention to the bodies that he has dumped down there. When Lou arrives home, she runs into Daisy who begins asking questions about why she was driving JJ's car that night. In order to silence her, Lou has sex with Daisy, who then answers the phone when Jackie calls from jail and tells her not to bother them. Instead, Lou Sr. bails Jackie out of prison and claims he can help her get out of this mess. Lou has continued seeing Daisy to find out what she knows, but when they return to Lou's house, Jackie shoots Daisy in the face, then runs. Lou hides the body behind the couch when FBI agents arrive to continue asking questions about JJ, who was one of their biggest informants on Lou Sr. They reveal they know that Lou used to be Lou Sr.'s right-hand man, so she'd have enough information to put him away. Once they leave, Lou Sr. calls to tell Lou that he has Jackie and plans to pin the double homicide on her. Lou threatens to reveal everything to the FBI if he does that, so he sends over a corrupt policeman to kill her. She knocks him out, then goes to rescue Jackie, but runs into Beth who attacks her for killing JJ. Jackie is tied up in the garage, and after initially running from Lou, they embrace and make up. Lou sends Jackie off towards the car, while she goes to confront her dad, but he arrives and shoots Lou in the leg and prepares to kill her. Jackie, empowered by her love for Lou, appears as a 60ft. woman and pins Lou Sr. to the ground. Lou puts a gun in his mouth, but decides to leave him for the incoming police. The two girls drive into the desert, but Lou hears thumping from the bed of the truck and discovers that Daisy is still alive. As Jackie sleeps, Lou strangles Daisy and drags her body into the desert.