Roger and Alison are stepbrother and sister. Alison's father died and her mother Margaret has married Clive, a businessman and former RAF officer. Clive's former wife was notoriously unfaithful, bringing shame to the family and a particular kind of pain to his son, Roger. To bond the new family together they are spending a few weeks of the summer in an isolated valley in Wales, a few hours' drive from Aberystwyth. They occupy a fine house formerly owned by Alison's father, subsequently transferred to her to avoid death duty. He in turn inherited it from a cousin, Bertram, who died there in mysterious circumstances around the time Alison was born.
With the house comes Huw Halfbacon, also known as Hugh the Flitch, a handyman and gardener. He is the last of the original domestic staff to remain at the house. The former cook, Nancy, had left to live in Aberystwyth but is offered a substantial amount to come and resume her duties. With her comes her son Gwyn. He has never seen the valley before but knows everything about it, courtesy of his mother's stories. Someone else who knows a lot about the place is Huw, but he is very mysterious about it. Oddly, Nancy has told Gwyn nothing about Huw.
Alison hears scratching noises in the attic above her bed and persuades Gwyn to investigate. He finds stacks of dinner plates with a floral pattern. When he picks one up, something odd happens. He almost falls through the ceiling while simultaneously Roger, lounging by a large flat stone near the river, hears a scream and seems to see something flying through the air toward him. The stone is known as the Stone of Gronw. It has a hole neatly bored through it, and legend says that Lleu killed Gronw by throwing his spear clear through the stone that Gronw was holding to shield himself.
Alison begins behaving peculiarly. She traces the pattern on the plate onto paper and folds the result to make an owl. Finding out that Gwyn has been in the attic, Nancy demands that Alison give up the plate. Alison asserts she has no right to it, and eventually produces a blank white plate. She maintains that the pattern disappeared from the plate. Alison becomes obsessed by the plates. One by one she traces them and makes the owls, and one by one the plates become blank. The owls themselves disappear, though no one cares at first about some folded paper models.
In the house's billiard room part of the wall has been covered with pebble-dash. Bit by bit this begins to crack and fall away, exposing first a pair of painted eyes, and then an entire portrait of a woman made of flowers. Tensions between the occupants of the house began to rise. Gwyn is intelligent and wants to further his education, but Clive expresses a stereotypical clannish closed-ranks attitudes of the upper middle-class towards him. Roger begins to feel hostile despite his initial friendliness. Eventually he takes to ridiculing Gwyn's efforts to improve himself with elocution lessons on gramophone records, calling them "improve-a-prole." Alison seems friendly to Gwyn and the two go on long walks together. She has visions where she sees herself next to him, even though he is some distance away. Margaret, her mother, never appears but the need to keep her happy affects everyone else.
As the holiday slides into disaster, the British attitude of "seeing it through to the end" prevails, even though Clive could pay off all the staff and leave at any time. Nancy repeatedly threatens to quit, and is repeatedly mollified with crisp banknotes. She constantly warns Gwyn to stay away from the others, lest he be taken out of school and forced to work in a shop to earn some money. Nancy gradually reveals her resentment of Margaret and Alison, as she once expected to marry Bertram and should have been mistress of the very house she toils in.
Competition between the boys for Alison is, at most, a subtext here, although Garner's television script is much more overt about Gwyn's attraction to her, despite his poverty. Alison, notwithstanding any attraction to him, would rather keep her privileges, such as her tennis club membership, than cross her mother. This is something she eventually admits to Gwyn.
The mysterious Huw presides over this like some ringmaster at a circus, making strange pronouncements. There is a background of odd comments from the villagers, noises of motorcycles in the distance, sounds of birds, and mysterious noises from locked buildings.
Gwyn follows Alison on one of her inexplicable midnight walks. She is completing her tracing of the plates somewhere in the woods. Gwyn is harassed by columns of flame, which he tries to convince himself are merely marsh gas. He finds Alison when she has made the final owl, then escorts her back to the house. Huw is waiting, and greets them with the remark "She's come."
By this time the connection between the legend and the events is becoming clear. Gwyn tries to see things rationally. He attempts to run away, walking up the sides of the valley as the weather worsens, but is chased back by a pack of sheepdogs. Stealing Roger's hiking gear, he tries the other side of the valley, only to find Huw waiting for him. Huw tells him of the power that exists in the valley, how those of the blood have to re-enact the legend each time, and how Blodeuwedd always comes as owls instead of flowers because of the hatred. Huw, of course, is Gwyn's real father, so Gwyn is the next generation. Huw was responsible for Bertram's death, sabotaging the old motorcycle Bertram liked to ride around the garden, not realising he would take it out on the dangerous hill road. The dinner plates and the wall painting were done by Huw's ancestors, trying to lock up the magic in their creations, but Alison has let it loose again.
Huw directs Gwyn to a crack in an ancient tree where he finds various things, including a spear head. All the men of Huw's line come to this tree, where they leave something and take something. Gwyn removes a carved stone, leaving a cheap owl-decorated trinket. He tells Huw to give the stone to Alison. Gwyn is going to return to the house and leave with his mother, who has finally and irrevocably quit.
In a locked room, Roger discovers a stuffed owl, which Bertram shot in his own attempt to lay the ghost of Blodeuwedd, along with all the paper owls that have traced intricate patterns in the dust of a storeroom. He also finds the sabotaged motorcycle. Nancy charges in and wrecks the place, destroying the owl and then attempting to knock the very feathers out of the air.
Alison, having been given the stone by Huw, collapses and is brought into the kitchen, where she writhes in the grip of some force that makes claw marks on her skin. Huw begs Gwyn to comfort her, but Gwyn by now feels totally betrayed by Alison and can say nothing. A storm rages outside, branches smashing through the windows and skylight, a sound of owls and eagles pressing in on them. Feathers swirl in the room and trace owl patterns on the walls and ceiling.
Roger is desperate, abasing himself before Gwyn, but to no avail. "It is always owls, over and over and over," says Huw. Then Roger shouts that it's not true, that she is flowers. He yells this at Alison until abruptly all is peaceful, and the room is filled not with feathers but with petals.