After a long journey and a hard battle, the Spook and his apprentice, Tom Ward, have returned home. But the county has been overrun by enemy soldiers. The Spook's house is in ruins, the boggart protecting it has fled, and the malicious witches imprisoned in the gardens have escaped. Tom, Alice, and the Spook must flee, too, across the ocean to the island of Mona. And it's on Mona that this small band fighting against the dark will face an old enemy grown terrifyingly powerful. Will they be able to vanquish an evil that crawls beneath the ground itself? And at what cost?