Siblings Julian, Dick and Anne are at home with their cousin George and her dog, Timmy. The children are suffering coughs, which results in them being sent via a hired car to a farm named Magga Glen in the Welsh mountains to recover. The Five have nearly reached their destination when their driver takes a wrong turn, leading them to a building called Old Towers atop a desolate hill. They intend to ask for directions but are deterred by locked gates and a fierce guard dog. While exiting, the car inexplicably feels heavier, as if the brake is on. Upon reaching Magga Glen, they are greeted and fed by the owner, Glenys Jones. The next morning, the Five meet Mrs Jones's brusque son, Morgan, who is powerfully built. Although he says little, Morgan has a stentorian voice that he uses to call his seven dogs. When the Five go for a walk, Timmy gets into an altercation with three of the farm dogs and is bitten by one of them. Upset, George promptly resolves to leave the next day. Julian and Dick climb the hills and find the farm's guest chalet. They also encounter the shepherd's young daughter, Aily, with her dog, Dave, and her pet lamb, Fany. George is dissuaded from leaving when the children opt to spend the rest of their holiday in the chalet. They enjoy their first day there, but at night strange rumbling sounds, vibrations and a shimmering light emanate from the hillside. The next day, while out skiing, Julian and George see a woman's face in a window at Old Towers, but the building's caretaker curtly declares that he is the only resident. Aily befriends the Five and shows them a note, thrown from a window of Old Towers by the elderly owner, Mrs Thomas. The note states: "I want help. I am a prisoner here in my own house, while terrible things go on. They have killed my son. Help me, help me! Bronwen Thomas." Julian and Dick show the note to Morgan, who takes it from them and orders them not to interfere. His attitude causes the children to suspect he is a villain. Aily takes the Five to a pothole with a tunnel leading to Old Towers. They locate Mrs Thomas, who tells them some evil men murdered her son, Llewellyn, and are exploiting a rare metal beneath the hill in order to make bombs. Back underground, the children spot Morgan and the shepherd and observe the subterranean mining operation. All of them are caught by the villains, who are led by Llewellyn Thomas. Morgan breaks free and calls his dogs, who come to the rescue. Back at the farm, Julian telephones the police. Morgan later returns with the shepherd and says the villains have been jailed. The group then celebrates Morgan's birthday.
Characters
George – A short-tempered tomboy, often mistaken for a boy.
Timmy – George's dog.
Anne – George's younger cousin, a very domesticated girl.
Julian – Anne and Dick's older brother.
Dick – Julian and Anne's brother
Dr. Drew – Doctor who prescribes a holiday in the snow to recover from colds.
Jenkins – Gardener who is Mrs Jones’s nephew and proposes the Five visit Magga Glen.
