Alison and Roger are step-brother and sister - their parents have recently married and they are on a family holiday in Wales. Nancy, her son Gwyn and Huw Halfbacon, the gardener, are keeping house for them. One evening Alison hears a curious scratching coming from the loft; Gwyn investigates and finds an old dinner service - with which Alison becomes increasingly obsessed.

At night Roger hears noises coming from Alison's bedroom and investigates. Moaning in her sleep, Alison suddenly reaches out and scratches Roger's face. Something is going on and nobody knows quite what it is. One thing is certain: it isn't just their imagination.

Gwyn and Alison read The Mabinogion and Gwyn explains the ancient legend of Blodeuwedd and the Stone of Gronw; Blodeuwedd was created out of flowers as a wife for the hero, Lleu, but was turned into an owl after she had been unfaithful with a man called Gronw. Huw thinks there might be a connection between the plates and the legend.

Alison has hidden the plates but Gwyn follows her to her 'special little house' in the garden and discovers the hiding place. She tells Gwyn she cannot stop making paper owls - Huw believes the power of the valley is now possessing Alison, Roger and Gwyn.

Roger thinks he might have photographed a ghost from an old legend connected with the house. Nancy is bitter about the injustices that prevented her from marrying Bertram, a former owner of the house. Huw feels that Gwyn, Alison and Roger may be re-enacting the tragedy of the old legend and that all three are becoming possessed.

Alison and Gwyn discover that just before Bertram was killed, he gave Nancy the dinner service they found in the loft. Alison and Gwyn have arranged to meet secretly in the stables, but Roger attempts to persuade Clive to cut the holiday short. Alison hears strange noises coming from a locked outhouse.

Gwyn thinks the mysterious Huw knows much more about the accident than he admits. When Clive asks Nancy about the key to the locked outhouse, she turns on him and says she is leaving. Panic-stricken, Gwyn storms blindly through the house, stealing food, clothes and money, determined to lave the valley behind him for good.

Gwyn now knows who killed Bertram. Huw tells Gwyn that he is really his father, and that years before he and Bertram were in love with Nancy. Roger is determined he will break into the locked outhouse where he believes the mystery will finally be solved.