U Turn

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A drifter named Bobby (Sean Penn) is on his way to pay off a large debt of $13,000 to a gangster in Las Vegas when his car breaks down, forcing him to drive to the nearest town, Superior, Arizona. He takes the money with him but leaves his gun in the trunk of his car. While waiting for his car to be fixed by the town mechanic, Darrell (Billy Bob Thornton), he wanders around town where he meets Grace McKenna (Jennifer Lopez). Not realizing she is married, he hits on her and helps her carry some drapes to her car. She offers to take him back to her house where he can have a shower.
While in the shower, it is revealed that the "accident" that happened to his hand was in fact a punishment for the overdue debt â two of his fingers were cut off. After his shower he attempts to seduce Grace, who is cold to his advances. He goes to leave, saying he is not interested in playing games. The two then kiss, and are caught by Grace's husband Jake (Nick Nolte), who punches Bobby.
As Bobby is walking back to town Jake pulls up beside him and offers him a lift. After a casual conversation about Grace, Jake asks Bobby if he would kill her for a price. Bobby laughs this off and asks Jake if he is just trying to "rattle" him.
Later on when Bobby is in a convenience store, the store is held up. The robbers take his bag with all his money in it but the shop owner takes out a shotgun and kills them both, destroying most of the money in the process. Broke and stranded, Bobby frantically calls nearly everyone he knows trying to get money to fix his car. He even calls the gangster he owes money to asking him for money but the gangster angrily refuses. The gangster now knows where Bobby is and sends someone after him.
When Bobby is sitting in a diner ordering a beer, he is approached by Jenny (Claire Danes), which provokes the jealous rage of her boyfriend, Toby N. Tucker (Joaquin Phoenix). The resulting fight is stopped in time by the town Sheriff (Powers Boothe).
Bobby also goes back to the mechanic, where he finds that Darrell has done additional work and is asking a higher price. He also busted open the trunk of the car, meaning Bobby cannot access his gun. A confrontation breaks out which results in Darrell scratching up the hood of Bobby's car. Darrell says that he will continue working on the car and charge more and more for the work until Bobby has the means to pay him.
Desperate for money, Bobby approaches Jake about killing Grace. Jake tells him that he would need to take Grace out of town and push her off a cliff in order to make the murder look like a suicide. A dream sequence shows that Bobby could not go through with it and ends up sleeping with her instead. Grace reveals that she is not just Jake's wife; she is also his illegitimate child. Jake sexually abused her from a young age and then married her after her mother died. Her mother was found dead at the bottom of a cliff but her death was ruled a suicide; her death is eerily similar to how Jake told Bobby to kill Grace. Grace then asks Bobby if he would kill Jake, meaning the two of them could steal his money. Bobby initially refuses.
Still broke and aware that the gangsters will send someone to find him, Bobby attempts to buy a ticket out of town. Although he does not have enough money, the clerk gives him the ticket anyway after he becomes aggressive and hostile. After buying the ticket he sees one of the gangsters driving towards him. However, the Sheriff stops the gangster and arrests him for concealing a gun. Thinking he is safe, Bobby is then attacked by Toby, who takes Bobby's ticket, rips it up and eats it. Bobby loses his temper and beats Toby to a pulp.
Knowing he is out of options, Bobby agrees to kill Jake. When he breaks into Jake's house while Jake is having sex with Grace, he accidentally makes a noise. Jake goes downstairs to investigate and finds Bobby, who then tells Jake that killing him was Grace's idea and that he will kill Grace for just enough money to pay for a car. Grace overhears this, grabs a tomahawk and apparently attacks Bobby when he walks into the room. However, she is really waiting for Jake to return so that she can kill him. Jake enters the room and finds Bobby lying on the floor. Bobby is playing dead and he attacks Jake with a golf club and they both fight. Grace then hits Jake in the back with the tomahawk and during a struggle between Bobby and Jake, Grace hits Jake in the chest with the tomakawk. Bobby grabs the tomahawk and kills Jake by striking him in the chest. Bobby and Grace unlock the safe to find $200,000. They then have sex in front of Jake's dead body.
Afterwards, Bobby goes to Darrell to get his car back. When he returns, Grace's car is gone. Bobby thinks she has run off with the money but she merely hid the car in the garage. As they drive off however, they are pulled over by the Sheriff, with whom Grace has been having an affair. Grace appears to turn on Bobby, blaming him for Jake's death. However, she shoots the sheriff and puts him in the trunk with Jake.
As Bobby and Grace are dumping the cadavers, Bobby elbows Grace in the nose and takes back his own gun, telling her that he does not trust her and that when they reach California they will split the money and go their separate ways. Grace says that when she blamed Jake's death on Bobby, she was merely baiting the Sheriff so she could kill him, much like Bobby baited Jake when he said he would kill Grace. As Bobby stands over the edge looking down at Jake, Grace pushes him over the cliff, severely injuring him. She gets in the car to drive off before realizing that Bobby has taken the keys. Bobby pleads with her to come down and help him. When she gets close enough however, he strangles her. He then makes the grueling journey back up the cliff with a broken leg. He starts the car, only for the radiator hose to burst.

Question: Who has Grace been having an affair with?
Answer: Sheriff