After a weekend of emotional honesty at an Esalen-style retreat, Los Angeles sophisticates Bob and Carol Sanders (Robert Culp and Natalie Wood) return home determined to embrace complete openness. They share their enthusiasm and excitement over their new-found philosophy with their more conservative friends Ted and Alice Henderson (Elliott Gould and Dyan Cannon), who remain doubtful. Soon after, filmmaker Bob has an affair with a young production assistant on a film shoot in San Francisco. When he gets home he admits his liaison to Carol, describing the event as a purely physical act, not an emotional one. To Bob’s surprise, Carol is completely accepting of his extramarital behavior. Later, Carol gleefully reveals the affair to Ted and Alice as they are leaving a dinner party. Disturbed by Bob’s infidelity and Carol’s candor, Alice becomes physically ill on the drive home. She and Ted have a difficult time coping with the news in bed that night. But as time passes they grow to accept that Bob and Carol really are fine with the affair. Later, Ted admits to Bob that he was tempted to have an affair once, but didn’t go through with it; Bob tells Ted he should, rationalizing: “You’ve got the guilt anyway. Don’t waste it.”
During another visit to San Francisco, Bob decides to skip a second encounter with the young woman, instead returning home a day early. When he arrives, he discovers Carol having an affair with her tennis instructor. Although initially outraged, Bob quickly realizes that the encounter was purely physical, like his own affair. He settles down and even chats and drinks with the man.
When the two couples travel together to Las Vegas, Bob and Carol reveal Carol’s affair to Ted and Alice. Ted then admits to an affair on a recent business trip to Miami. An outraged Alice demands that this new ethos be taken to its obvious conclusion: a mate-sharing foursome. Ted is reluctant, explaining that he loves Carol “like a sister,” but eventually acknowledges that he finds her attractive. After discussing it, all four remove their clothes and climb into bed together. Swapping partners, Bob and Alice kiss fervently, as do Ted and Carol, but after a few moments all four simply stop.
The scene cuts to the couples walking to the elevator, riding it down, and walking out of the casino hand-in-hand with their original partners. A crowd of men and women of various cultures and races congregate in the casino parking lot, wherein the four main characters exchange long stares with each other and with strangers, reminiscent of the non-verbal communication shown in the early scene at the retreat. Over this final scene, the film’s theme song reminds the viewer that “what the world needs now is love.” The credits roll as the couples look into each other’s eyes.