Born to twin sisters on the same day in London, France and Margaret are more like brother and sister than cousins. The close relationship between the cousins is suspended when Margaret is sent abroad for schooling. Now a young woman, Margaret returns to London and resumes her intimate connection with France. Fearing that their relationship could become incestuous, France decides that he and Margaret should take lovers, and chooses Fred, a Swedish architectural student, to be his cousin's lover. For his mistress, France picks Manny, an American who has had many lovers. The foursome spends a romantic summer interlude at an estate in Spain, where Margaret beds Fred while France takes Manny. Once back in London, however, Manny senses that France is in love with Margaret and begins to suspect that he is using both Manny and Fred to suppress his own feelings. To spare the unsuspecting Fred, Manny seduces him. Upon discovering that she is pregnant, Manny is uncertain about the paternity of her baby, but nevertheless names Fred as the father. Manny's former lover, a middle-aged millionaire named George, offers to throw Manny and Fred a lavish wedding reception, but when Manny gives birth on the day of the reception and is forced to miss the celebration, Margaret and France find themselves the sole attendees. Following the wedding and birth, the four friends decide it is best to end their close relationship, and so Fred takes Manny and their baby to live in Sweden, while Margaret leaves for Rome to study art and France stays behind in London. Some time later, Margaret returns to London after the end of a disastrous love affair, and France suggests that they visit Fred and Manny in Sweden. There they find that Manny and Fred are trapped in an unhappy marriage. Fred, who is still in love with Margaret, takes her on a tour of Sweden while Manny and France rekindle their love affair.