The children are suffering coughs, which results in them being sent 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 on a desolate hill.

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, stating she is a prisoner and asking for help.

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.