‘Quick, William, get out, get out!’ shouted Dylan in a panic-stricken voice. William turned and saw the water, waves high, rushing towards him, almost upon him. He scrambled out of the canoe, almost falling in the water, in a frantic effort to get to the shore and to safety. He flung himself the last couple of feet and landed in a heap on the beach. Moments later, the waves hit the canoe, and it rocked violently and tipped over. Luckily, it was still secured to the shore by its mooring rope, or it would have been swept away. The waves continued to rush up the lake towards the top of the valley, the lake had gone from a millpond to a raging torrent in a matter of moments.

Dylan, William and Mary have moved to a new home in a beautiful valley in deepest Wales with their parents. The two boys love exploring, usually leaving their younger sister, so they think, safely at home. However, little did they know, Mary has been off exploring on her own and has made an amazing discovery in the mysterious Tower that stands in the lake. When strange things happen and William keeps feeling he is being watched, only their little sister Mary knows the truth, and she is sworn to secrecy. When Mary is struck down with a mysterious illness, her new secret friends must find out the cause and the cure, before it is too late.