John, 17, and his sister, Julia, 16, live with their parents. They are two attractive, but inexperienced young people whose initiation into sex we are going to closely follow.

One day, Marina, 25, and her husband, Al, 28, a friendly couple spend the night at John and Julia's house as guests of their parents. Marina's magnetism soon captivates John, especially after two incidents - a shower scene and an intimate bedroom scene between her and Al - which John accidentally watches. Marina, aware of John's envious glances, follows him into his bedroom and seduces him. The next day she proceeds to seduce Julia too, when the latter comes back from school and finds nobody at home except Marina.

A few days later, John and Julia are invited to a party at Marina's house, which eventually turns into an orgy. Al takes Julia's virginity, while John desperately seeks the embraces of a wild Marina, with whom he is deeply in love. However, the next day, we find Julia still at Marina's house, being seduced by both her and Al. John, excluded from this peculiar triangle bursts into hysterical crying when nobody bothers to answer his desperate banging at the door.