Plot

In this modern sexual morality thriller, a pair of modern country couples lounge around, eyeing each other with heavy-lidded salaciousness, then pairing off illicitly, while one of the wives recounts to a detective how she accidentally toppled her husband over the rim of Croton Reservoir upstate New York. Or was it an accident?—Anonymous

A man and a woman run onto a high bridge that spans a dam. On a bet the man begins to walk on the edge of the narrow wall. He slips--or is pushed--and plunges to his death. In a police station later, Betty, the man's wife, reconstructs the events of the few weeks before the tragedy: Her childless marriage to Bob, held together paradoxically by infidelities, is further aggravated when an equally disillusioned couple, Ann and Alan, move into their suburban neighborhood. The quartet enter into a series of mate-swapping episodes and the arrangement holds their respective marriages together. Soon a third couple, Judy and Harold, who had a similar arrangement with Bob and Betty some years earlier, arrive on the scene. In a drinking spree one afternoon, Harold dares Bob to walk the wall of the bridge over the dam. Bob accepts the bet, wagering Betty against Harold's $50. Though Harold wins, he faints before collecting his prize. Bob, taunted by Betty for losing his courage, persuades Alan to join him and Judy for a garden party. As intended by Bob, Ann and Betty stumble upon the threesome's lovemaking and their revulsion awakens in them a closeness that they have never experienced with any man. After listening to Betty's story, the inspector tells her what he believes happened: She and Bob have a drunken quarrel in their car over her emasculating behavior and her feelings for Ann. Already resentful of Betty's repeated references to his lack of manhood, Bob tries to walk across the dam but in the process falls to his death. The inspector points out to Betty that, since there is no mud on her shoes, she obviously never left the car; her guilt, if there is any, is moral, not legal. Leaving the police station, Betty is met by Ann. Upon noticing there is mud on Ann's shoes, Betty suddenly realizes what actually happened on the bridge. Ann suggests she will keep the shoes, and Betty realizes that her tie to Ann is the knowledge of their mutual implication in Bob's death.