Seth Winnick, a wealthy sitcom writer-producer with an Elvis fixation, meets Chelsea Turner, an interior decorator, at the wedding of their best friends, Larry and Holly Garnett. Although Seth has brought another woman as his date, the two click and quietly slip away from the reception together. On Holly’s advice, Chelsea sets a quirky rule: she won’t sleep with a man unless they’ve shared at least three meals. Seth artfully accelerates their evening, racing through activities and, by the third meal, Chelsea offers to sleep with him but only if they achieve complete exclusivity. Though unsettled, Seth agrees, and they end up sharing a wild night of sex in Chelsea’s showroom.
At first, the pair seem happy, but Chelsea is determined to move their relationship forward and marry quickly. She proclaims her love after only four weeks; when Seth hesitates to mirror those feelings, Chelsea explodes and nearly ends things. To stop her, he blurts out that he loves her in a moment of desperation. When Chelsea’s cat goes missing, she uses it as a pretext to move in with Seth, despite his reservations—only for the cat to reappear days later.
On a Valentine’s Day getaway, Chelsea anticipates a proposal, but Seth presents jewelry instead. He explains that he believes it’s too soon to get engaged, but Chelsea pushes on, and he promises to propose in one year if things feel right. Weeks later, Chelsea creates a scene at the filming of Seth’s sitcom upon seeing he has cast an attractive actress, Tawny, despite Chelsea’s prior request not to. Larry convinces Seth not to end the relationship and to try to work things out.
Over the next year, the couple fights constantly but reconciles with intense, make-up sex. On Valentine’s Day, Seth again delivers jewelry instead of a proposal, and Chelsea reacts with fury. He argues that their volatile pattern makes him uneasy about proposing. Chelsea ends the relationship and tosses the diamond earrings into the Pacific Ocean.
Chelsea then files a palimony lawsuit, threatening to seize Seth’s house and half of his assets. Seth’s lawyer warns that the case could cost him hundreds of thousands of dollars. Chelsea moves back into his house, intensifying the drama by sabotaging his life: filling the place with rescue cats to trigger his cat allergy and swapping his shampoo for hair-removal cream. In response, Seth kidnaps Chelsea’s cat and uses it to lure her into a plan to push her off the edge of the Santa Monica Pier into the filthy ocean water.
They briefly reach an uneasy truce, sealed with angry hate sex, but Chelsea’s lawyer persuades her to renege and obtains a restraining order to force Seth to move out. Pushed to his limit, Seth kidnaps Chelsea and takes her to a planetarium. She fears he intends to murder her, but instead he proposes. When she won’t commit, he shows her a ring she had previously demanded, and she radiantly accepts after it is appraised as legitimate. He then sells his house to begin anew and convinces Chelsea to marry him on the Las Vegas Strip, planning to renew their vows later with a grand ceremony. He even arranges for the wedding to proceed with no family attendance, save for an Elvis impersonator.
In the airplane bathroom, Larry pleads with Seth to call it off, but the ceremony goes forward. During the vows, Seth drops a bomb: “I don’t think so,” revealing that the engagement was a ruse. Chelsea had dropped her lawsuit and moved out, and Seth discloses that his dentist and his assistant posed as a married couple to buy the house, which they will now sell back to him. He also admits to having bribed a jeweler to appraise Chelsea’s glass engagement ring as a real diamond worth $75,000. A furious Chelsea grabs the chapel security guard’s gun and shoots Seth, who is dancing triumphantly down the aisle. Chelsea is arrested by the Las Vegas police, and an ambulance carries Seth away as they scream insults at each other. Chelsea vows to sue him for breach of promise. Seth, reveling in the chaos, proclaims he will be “full-on Liberace gay” from now on, while Larry and Holly sigh that they need new friends. As the ambulance drives off, a groping encounter with a male paramedic leads Seth to recant his coming out. In the squad car, Chelsea tries to propose to the arresting officers, only for one to reply, “Lady, don’t make me gag you.”
