Substitution (1970)

When lovemaking of a married couple, Henry and Alice becomes strictly routine, the husband takes Fred's, his best friend advice and goes on a pilgrimage to see the all-knowing Maharaji. The first night of his return, he anxiously waits for his wife to emerge from the bathroom so they can begin the night's festivities. Much to his surprise, the woman who steps through the door is a gorgeous seductress. This leads to a passionate and happy night. The next night, another lovely woman enters his room and again, romantic fireworks light up the room. And so it goes with each night becoming more adventurous and exciting than the last. The secret of the couple's newfound romantic bliss is revealed at the end.