Friends don't let friends mess around on the side in the comedy How to Save a Marriage, though it quickly becomes obvious that making this happen isn't as easy as it may sound. When David Sloane (Dean Martin), a single lawyer with an eye for the ladies, learns that his best friend, Harry Hunter (Eli Wallach), has been cheating on his wife, he finds himself worried for his pal and decides to help him get his life back on the straight and narrow. David thinks that Harry is having an affair with his secretary, Carol Corman (Stella Stevens), so David begins romancing her himself, and soon he has Carol installed in a cozy love nest of their own. However, David has been using his seductive powers on the wrong woman; Harry has actually been fooling around with Muriel Laszlo (Anne Jackson), who lives nearby. When David tells Harry that he's stolen his mistress away from him, Harry isn't sure what's going on, but the message has the intended effect, and Harry patches things up with his wife Mary (Katherine Bard). But when Carol and Muriel compare notes, they decide that something is fishy, and the two draw up a series of demands -- Muriel insists that Harry leave Mary and marry her, while Carol wants David to make it legal with her.