Laura (Lara Wendel) and Fabrizio (Martin Loeb) have been meeting every summer in the forest by her parent's summer home. Fabrizio is a solitary boy with only his dog for company; Laura a sweet, but unconfident child. This summer new aspects enter into their story as both are growing up. The film represents them as part child, part adult. Part naive, part knowing. Laura is falling in love with Fabrizio, while he displays a new sexual awareness of her masked by his malice.
Fabrizio becomes inexplicably cruel. He accelerates his unwarranted torment of Laura in many ways, including tying her up and putting a snake near her and killing a pet bird she is fond of. Fabrizio prides himself on being 'king of the forest' and rubbishes Laura's tender attempts to be his queen. One day they climb the "Blue Mountain", a mysterious tall mountain at the forest's edge and discover ancient building ruins. Exploring these they find a cave. Inside, Fabrizio seduces Laura.
Fabrizio's cruel streak is boosted by his new sexual confidence. At one point he virtually forces himself on Laura, much to her upset. He does relent when she makes it clear she wants Fabrizio to be gentle with her, which he ridicules.
Things develop further when they meet Sylvia (Eva Ionesco). Unlike the previously virginal Laura, Sylvia is confident and assertive. Fabrizio develops a fascination with her, eventually bribing Laura to fetch her to the forest to join them in play. Sylvia aware of Fabrizios interest in her, asserts herself in his affections, quickly replacing Laura and demoting her to servant and victim, which Fabrizio takes delight in. Laura, reluctant to leave her old friend and new lover, stays and becomes the target of the duo's ever progressing cruelty. At one point, they both 'hunt' Laura with bows and arrows and at another, pretend to throw her off a high ledge. They make love in front of her, insisting her punishment is that she must watch, leaving Laura confused and heartbroken.
At the end of summer, with the girls talking about returning to school, Fabrizio becomes pensive and agitated. He insists on taking Sylvia to the ruins for the first time. All three of them go into the cave to escape a thunderstorm and Fabrizio again pretends they are lost as he did with Laura. Sylvia breaks down sobbing for her mother, all traces of her confidence and maturity lost in the fear of being in the cave. Fabrizio repeatedly begs Sylvia to stay with him forever. In the morning, Sylvia is still lost in the cave and further rejects the desperate Fabrizio and his pleas to stay with him. She becomes hysterical and he kills her with a knife, feeling it is the only way he won't lose her. He stays with the dead body and gives Laura the flashlight telling her she knows the way home and Laura reluctantly leaves.