Chelsea changes from her bridesmaid dress into a sexy outfit and tells him that she has only one rule: she won't have sex with a guy until they have had at least three meals together. Seth proceeds to speed date through the evening ensuring that they actually eat three meals. They end up in the showroom of Chelsea’s job having sex on a four poster bed until Chelsea’s boss interrupts them and presumably fires Chelsea.
Seth then hires Chelsea to decorate his new house and through a series of manipulative events, Chelsea ends up living with and mooching off of Seth. She tries to get him to marry her through a combination of skimpy outfits and wild mood swings. When she finally realizes he's not going to propose, she turns into a vengeful harpy and makes his life hell.
Seth soon breaks up with Chelsea. However, she refuses to leave his home and instead sues Seth for palimony. Seth and Chelsea then proceed to engage in a series of actions in which each character tries to get even with the other. For example, Seth pretends to hurl Chelsea's cat over the Santa Monica Pier and farts in their still shared bed, while Chelsea puts depilatory lotion into his shampoo and breaks his treasured collection of Elvis plates.
After a palimony hearing ends in disaster, Chelsea tells Seth she's going to a spa. Seth follows her to the spa and cancels her room and treatment reservations to torment her. While there, Seth convinces Chelsea to accept a $100,000 settlement after they are both pushed into a swimming pool and have begun to engage in "hate sex" on the beach. However, upon their return, Chelsea manages to have Seth evicted from his home and demands a larger settlement totaling $200,000. Banished to a cheap motel room, Seth realizes that the only way out of this mess is to actually marry Chelsea. He kidnaps her using chloroform, takes her to the Griffith Observatory and proposes marriage. Chelsea eagerly accepts.
The final scene takes place in a Las Vegas wedding chapel altar complete with an Elvis preacher, their best friends (Bellamy & Banks), and a trio of sequined backup singers. When it is Seth's turn to say "I do," he says instead "I...I...I don't think so." Everyone is surprised and he explains that this shenanigan was a hoax to get Chelsea to dismiss the lawsuit and to regain his home. Prior to leaving for Las Vegas, Seth (with Chelsea's knowledge) sells his house telling his "fiancee" that he wants a fresh start in a new home with her. In actuality, the buyers were his dentist and his dental assistant who plan to sell the home back to Seth forthwith. As he is currently not the owner of this home, both he and Chelsea have no legal rights to the property.
As Seth leaves the chapel, Chelsea grabs a security guard's gun and shoots Seth in the butt. The film ends with Chelsea being arrested and Seth on a gurney rolling into an ambulance. Chelsea is calling him an "asshole" and he is screaming in return, "Back to hell demon! Back to hell!"