To help best friend Harry Hunter (Eli Wallach) save his marriage, lawyer David Sloane (Dean Martin) agrees to seduce Muriel (Anne Jackson), Harry's mistress. What David doesn't realize, however, is he's wooing the wrong woman. Having mistaken secretary Carol Corman (Stella Stevens) for Muriel, the confirmed bachelor informs Carol of a vow he made to his fictional late wife never to remarry. Not wanting to be thought of as somebody's ex-mistress, Carol ropes David into a mock marriage with the promise of a quickie divorce. The morning after the wedding, however, David realizes if he's to save anyone's marriage, it'll be his own.