Just as English civil servant Henry Brittingham-Brett explains to his parents that he is no longer involved with his erstwhile girl friend Susan, she bursts into Henry's office and lavishes affection on him. Susan explains to the Reverend and Mrs. Brittingham-Brett that she is married to Henry's best friend, Sir Philip Ashlowe, a British nobleman who has little time for her after attending to his government affairs. When Philip arrives home that day, Susan complains about the effect his work commitment has had on their relationship and persuades him to take a party of friends for a vacation on their yacht. Having invited Henry and his parents to dinner that evening, Susan shows the guests home movies of her and Henry. Henry's parents are disturbed when they see Susan kissing Henry in nearly every scene and even more disturbed by Philip's naive suggestion that his best friend and wife are only having "good clean fun." Later, Henry's mother suggests to the Reverend that Susan is using their son to incite Philip's jealousy. Back at the Ashlowe's, Susan uses her sexy negligee to arouse Philip's interest in a romantic evening, but he is too busy to notice and kisses their dog Nelson instead of Susan. When the couple set sail on the yacht, Susan hopes for some time alone with Philip but finds that her industrious husband is too consumed with helping out on board at all hours. Needing a diversion, Susan calls Henry and entreats him to profess his love for her in a rendezvous on the deck. After days at sea, a violent storm sinks the yacht, marooning Philip, Susan, Henry and Nelson, who became separated from the other passengers, on an uninhabited tropical island. The practical Philip immediately begins building a big hut for him and Susan and a little bachelor hut for Henry. Susan secretly hopes that life on the island will provide more time to be with her husband; however, Philip is completely preoccupied with the building construction. He even rigs an intercom system between the huts with conch shells. Henry's attempts to aid Philip are clumsy and ignorant, causing both Susan and Philip to scoff at him. Susan takes over gathering the oysters, preparing meals and washing the clothes wearing only her black undergarments, saving her evening gown for meals. On the eve of the huts' completion, Susan sees her chance finally to be alone with her husband and invites him to the hut for romance. Jealous of Susan's affection for her husband, Henry distracts Philip with a game of chess. By the twenty-sixth day on the island Henry is so desperate for female companionship that he kisses Susan, then proposes that they have an affair. Susan, far from being shocked, concludes that Philip would never suspect them because he takes her for granted, but refuses Henry anyway. Sulking, Henry threatens to tell Philip that he loves Susan and offer a "lend-lease" situation. Since Philip has been generous enough to share his shoes, Henry concludes that Philip might lend him his wife as well. Infuriated by the immoral suggestion, Susan warns that the proposal will only ruin their friendship. Later that day, Susan is eavesdropping when Henry tells Philip that they should share Susan. Philip thinks the suggestion is deplorable, but Henry argues that Philip neglects his wife and claims that he has been Susan's lover for several years. When Philip protests that Susan is too much of a child to have had a lover, Henry falsely gives Philip his word of honor that the affair has taken place. Susan resents being referred to as a child and is incredulous that the suggestion does not anger Philip more. When Susan denies having an affair with Henry, Philip, the logical diplomat, resolves to find a solution to the dilemma. As captain of the ship, Philip claims he has the power to perform both marriages and divorces and asks Susan to state her case for divorce. Susan alleges neglect and indifference, citing Philip's inability to recall the shade of the negligee with which she last tried to seduce him. Philip concedes to a divorce and suggests that Susan use her conscience to decide the fate of their relationship. Leaving Henry alone at the big hut with Susan, Philip moves to the little hut with Nelson. Later that night, Philip sends Nelson back to the big hut, knowing the dog will bark Henry out of the couple's home and force him to sleep outside. Soon after, when Philip calls with the description of the negligee in question, he once again endears himself to Susan. The next morning, the competition between the two men has rejuvenated Philip's interest in his marriage. He and Susan are busy adoring one another, when a man dressed as an island native surprises the group and ties up Philip and Henry. They try to reason with him, but the native utters gibberish and then drags Susan to the big hut. Once inside the hut, Susan falls, causing the man to exclaim in Italian. Susan then recognizes the man as Mario, the yacht's chef, who concocted the scheme to satiate his "primitive desires." When Susan laughs at Mario's explanation, Henry and Philip assume she is enjoying her captor's attentions and blame each other for her infidelity. Susan is pleased to see that she has finally riled her husband's anger when she exits the hut. Suddenly a ship's horn sounds off shore, causing Mario to scream in English, revealing his true identity to the men. Once safely on board the ship, Henry finally concedes that there was no affair. Back in London, Henry, still assuming the divorce was valid, arrives one night at the Ashlowe house to ask Susan to marry him but finds Philip and Susan in their nightclothes preparing to retire. Upon seeing that Susan is knitting a baby sweater, Henry understands the nature of his defeat and exits the house with a barking Nelson in tow.
