The film centers on two main characters: Lazarus Redd (Samuel L. Jackson), a deeply religious farmer and former blues guitarist, and Rae Doole (Christina Ricci), a young sex addict. Lazarus' wife and his brother were having an affair, which has left him bitter and angry. Rae's boyfriend Ronnie Morgan (Justin Timberlake) leaves for deployment with the 196th Field Artillery Brigade, Tennessee National Guard, and in his absence, she indulges in bouts of promiscuity and drug use. During one of Rae's binges, Ronnie's friend Gill Morton (Michael Raymond-James) tries to take advantage of her. She laughs at his advances, comparing him unfavorably with another man, and he severely beats her. Believing she's dead, Gill dumps Rae and leaves her for dead in only a shirt and panties by the side of the road and drives away.
Lazarus discovers Rae unconscious in the road the next morning and brings her home to nurse her back to health. Lazarus goes to see Tehronne (David Banner) - the man who Lazarus thought had beaten her - and learns of her promiscuity. Over the course of several days, Rae, delirious with fever, occasionally wakes up and tries to flee from Lazarus. He chains her to the radiator to keep her from running away. After Rae regains her wits, Lazarus announces that it is his spiritual duty to heal her of her sinful ways and refuses to release her until he does so. Rae makes several attempts to escape, and even briefly has sex with a teenage boy who helps out on Lazarus' farm.
She eventually comes to tolerate her position. Lazarus buys her a conservative dress to wear, plays the guitar for her, and feeds her home-cooked meals. Lazarus' pastor and close friend, R.L. (John Cothran, Jr.), visits Lazarus at his house and discovers that Lazarus is imprisoning Rae. The pastor tries to reason with Lazarus and the group shares a meal.
Meanwhile, Ronnie returns to town after being discharged from the National Guard due to his severe anxiety disorder. While searching for Rae, who has disappeared, he meets Gill, who informs him that Rae cheats on him whenever he is out of town. Ronnie attacks Gill, steals his truck, and continues searching for Rae.
In the morning, Lazarus frees Rae, having decided that he has no authority to pass judgment on her. Rae chooses to stay with Lazarus of her own will. Later, Rae and Lazarus take a trip into town, where Rae confronts her mother (Kim Richards) about the sexual abuse she suffered at the hands of her mother's partner. Meanwhile, Lazarus has formed a budding romance with the local pharmacist, Angela (S. Epatha Merkerson). He plays a blues concert at a local bar, which Rae attends. Ronnie spots Rae and follows her to Lazarus' house. He confronts the pair with a pistol, but Lazarus talks him down and summons the pastor. Ronnie and Rae decide they are stronger together than apart and get married. While driving away, Ronnie suffers from a panic attack again and Rae begins to have one of her spells, but then they pull themselves together, and resolve to take care of each other.

Question: Why does Lazarus chain Rae to the radiator?
Answer: Lazarus was afraid that Rae would run away.  
Alternative answer: To keep her from running away.