Holidaying in rural Wales, Alison, mother Margaret, new stepfather Clive and his son, Roger are staying at a manor house formerly owned by Alison's uncle Bertram. Alison befriends the cook's son Gwyn. Together, they discover a set of dinner plates with a floral pattern, which Alison traces and forms into paper owls. Gwyn's mother Nancy is furious and demands the plates be returned. At night, Alison becomes possessed and as Roger looks in on her, she scratches his face.
The half-crazed gardener, Huw, tells Gwyn of an ancient legend and hints that the three teenagers may be in danger of re-enacting the story. Alison is acting strangely as she becomes taken with the female character of the myth. In the billiard room, Clive is astonished to see the wood-pannelling on the wall crumbling away to reveal a picture of a woman made of flowers.
Alison has hidden the plates away in a hut in the forest and becomes obsessed with the need to trace the pattern. Gwyn joins her in the hut and they spend the night there. In the morning, Huw rambles on to Gwyn that he was also one of three caught up in the legend and claims he is lord of the valley.
Roger is disturbed by some photographs he has taken in the valley which seem to depict a ghost. Nancy tells Gwyn that she had an affair with Bertram and bemoans the fact that she should have been the owner of the house rather than Margaret. Margaret meanwhile has forbidden Alison to see Gwyn, but while out walking, Alison meets up with him. Unknown to them, Margaret is watching through binoculars.
Alison and Gwyn become closer as they walk up the mountain, while Roger is at home feeling increasingly depressed and expresses his desire to leave to Clive. Alison and Gwyn promise to meet up every day out of sight of their parents at an agreed time, but on her return to the house, Alison is forbidden to see him. A few days later, Roger, Alison and Clive discover a locked door in the stables and wonder what's inside. Alison then realises that she is in her and Gwyn's meeting place and as he steps out of hiding, she is forced to snub him.
Gwyn is furious at the treatment he has been shown by Alison and is further upset by his mother. In an attempt to bond, Alison and Roger talk together and she reveals some of the things Gwyn had told her while on the mountain. Later, Gwyn corners Alison and forces her to discuss what's going on between them. Roger shows up and mockingly tells Gwyn he knows his secrets. Gwyn is enraged and storms away, determined to leave the valley and everyone behind.
Gwyn has spent the night on a mountain when Huw catches up with him. Huw explains that the legend was re-enacted by him, Bertram and Nancy and is now being enacted again by Gwyn, Roger and Alison and can only end in pain. Gwyn is not interested until Huw reveals that he is his father. At the house, Roger decides to find out what is in the stable and goes to force open the door.
On opening the door, Roger finds a table full of paper owls - the same as Alison had been making, but these were put there by Nancy. At the house, Gwyn has returned but is told by Nancy they are leaving. Struggling through the heavy rain with their cases, Gwyn tells her he is staying and she leaves him behind. Alison has become completely possessed by the legend and collapses. Huw takes her inside as scratches and feathers appear all over her. In panic, Roger attempts to help, but Gwyn refuses. Finally, Roger manages to talk Alison out of her state by realising the pattern on the plates was of flowers, not owls.