At an Alabama truck stop, truck driver Joe Majors walks in on a woman, Julie, being choked and seemingly assaulted by another man. He grabs the man and pulls him off Julie. However, Julie goes on to explain that she had asked the man to choke her. She relays a childhood experience during which she nearly drowned and had an out-of-body experience which allowed her to enter the spirit world. Since her daughter Billie was recently left in a coma after a motorcycle accident, Julie, desperate to save her, was trying to find Billie's spirit in the nether world to return it to her body when Joe walked in on her being choked. Joe – whose wife and daughter tragically died – takes Julie to the hospital to witness Billie go into cardiac arrest. Julie makes Joe choke her and enters the nether realm to lead Billie's spirit back to her body. Billie survives.

Joe accompanies Julie home and, even though Joe feels and looks as though he's been on the road for three days, they have sex. He stays with Julie for the next few days and is fired for not completing his duties in a timely fashion. He laments that a man without a truck is not a man. Billie is discharged from the hospital. While her injuries are healing, her demeanor seems odd to Julie and to her friends Mike and Rick. She also makes sexual advances to Joe, who is confused but not disinterested. While Julie drives into town to retrieve Joe's belongings from his former employer, Billie reveals to Joe that she is not in fact Billie but the spirit of his deceased wife, Mary, who has taken over Billie's body. Joe is quick to believe her and starts to engage in frisky activities with her, rushing to cover up their exploits when Julie returns home.

Julie is suspicious of Joe's and Billie's behavior and speaks to a nurse from the hospital who is knowledgeable about the logistics of the spirit world. She tells Julie that the spirit that returned to Billie's body is not Billie's and suggests that it is likely that of Joe's dead wife. Meanwhile, Joe and Billie/Mary have intense intercourse at home. During his coitus with Billie, Joe is drawn into a dreamlike memory or alternate world in which he is having sex with Mary. Julie returns home and Joe panics, while Billie assures him that all he's hearing is the cat. He astutely objects, "You don't have a cat," just before Julie walks in on them in the act. Julie is furious and demands that Joe help her expel Mary's spirit from Billie's body. Billie/Mary knocks Julie unconscious to stop her and asks Joe to run away with her.

Billie and Joe intend to rob Mike and Rick. Joe knocks out Mike (who has affections for Billie) and gets into a scuffle with Rick, fatally shooting him by accident. Billie/Mary takes the cash she finds at the house and takes Joe back to the ruins of the house in which Joe and Mary used to live with their daughter Sarah until Mary and Sarah died in a tragic fire allegedly caused by accident. Meanwhile, Mike confronts Julie, who tells in him about Billie's and Mary's spirits being swapped. Joe gets emotional when he finds Sarah's jack-in-a-box as he still blames himself for her and Mary's deaths. Joe and Billie/Mary vow to rebuild their relationship, but Mike and Julie walk in on them and hold them up at gun point. Billie/Mary draws a gun on Julie and, in a shocking plot twist, admits to killing Sarah before setting the house on fire on purpose as she says she is "not good on her own". Julie walks towards Billie/Mary and is fatally shot by he