Plot

Unhappy couples fall apart and hop into other beds with other people.

Theater instructor Jerry starts an affair with Mary, and that starts a chain of events that affect their respective partners Terri and Barry and other characters in the film, creating a web of relationships.—Anonymous

Set in an unnamed Midwestern American city, two urban, middle-class couples deal with their unhappy relationships and shamelessly lie and cheat on one another in their selfish quest for happiness. Jerry (Ben Stiller), is a loathsome, flirtatious theater instructor whom is married to Terri (Catherine Keener), a writer whom is alienated and unfulfilled with his macho love-making skills. Jerry and Terri have dinner with Mary (Amy Brenneman), a writer friend of Terri's, and Barry (Aaron Eckhart) a sad-sack business executive whom is oblivious to his wife's unhappiness with him. During dinner, Mary talks about writing for a local newspaper column about bickering couples and the troubles they have, while Barry does not think that other couple problems are anyone else. After dinner, the lothario Jerry secretly makes a discreet pass at Mary whom he asks her out on a date. Marry, out of frustration in her unsatisfied life with Barry, accepts.

The next day, Terri, after going to a local art gallery, meets and begins a secret romance with Cheri (Nastassia Kinski), a lesbian art gallery worker whom Terri feels satisfied with the lovemaking with her and of the quiet of it compared with Jerry's loud and macho performance.

Meanwhile, Cary (Jason Patric) is a mutual and caddish doctor friend of Barry's whom is also a devious, sexual predator whom targets naïve and emotionally vulnerable young women whom he picks up, has sex with, and dumps just for the sole purpose to watch them cry. Aware of the distance between Barry and Mary, Cary persuades Barry to leave his wife to have his own swinging, non-monogamous lifestyle that Cary has built for himself. Barry thinks that his marriage can be saved.

During Jerry and Mary's rendezvous at a local hotel, Jerry fails to get aroused during foreplay. As a result, he takes out his frustrations on Mary, believing that she has made him impotent. Angry and offended by Jerry's misogynist outburst, Mary abruptly ends their "affair." She feels more miserable a few days later, when Barry unwittingly takes her to the very same hotel room to rekindle their romance. Mary realizes that Jerry had told Barry about being in the room. Barry fails to understand Mary's unhappy attitude and thinks he might somehow be responsible for it.

Jerry, Barry, and Cary get together to work out and, in the steam room, Barry tries to get them to reveal their best sexual experiences. Barry tells them that he only feels satisfied with himself. Cary then delivers a chilling monologue about his best sexual experience: partaking in a gang rape 20 years earlier in forcibly sodomizing a male high school classmate in the locker room at his boarding school. Both Barry and Jerry are stunned but fascinated by Cary's sordid and evil story. When Barry tries to persuade Jerry to reveal his best sexual experience, Jerry refuses. After being goaded in the locker room, Jerry angrily responds that his best sexual experience was with Barry's wife. He then leaves, with Barry too stunned to respond. Cary, also caught off-guard, says: "That beats my story."

After returning home from the gym, Barry confronts Mary over dinner about her affair with Jerry just as Terri accidentally finds out about Jerry's indiscretion and eventually confronts him too while they are shopping at a local bookstore. Mary and Jerry are both unapologetic for their unfaithfulness and express dissatisfaction to both of their spouses. Terri accidentally reveals her own lesbian romance with Cheri, but does not display any guilt for her infidelity.

Jerry confronts Cheri at the art gallery over his wife's affair with her. Cheri also shows no remorse or regret for her relationship with Terri, or with interfering with Jerry and Terri's troubled marriage. Cheri tells Jerry that Terri can do much better than being with him.

Both of the couples split up. Terri moves in with Cheri, although she finds her emotional neediness irritating. Jerry continues his philandering lifestyle with his female theater students. Barry becomes miserable because he is no longer able to give himself an erection during masturbation. Mary is revealed to have moved in with Cary, who treats her as coldly as the other women in his life even though she is pregnant with his child. The film closes on Mary and Cary in bed, as Mary realizes that she is even more unhappy in her new relationship with the catty, heartless, misanthropic and misogynistic Cary than she had been with her clueless husband Barry.