Kevin knows that Tess is a Switcher, for he is one too able to take on any animal shape he chooses and to think, feel and live as that creature. Kevin persuades Tess to join him as a rat, and the two make their way beneath the snowbound city through the underground network to see Lizzie, a strange old woman who tells the children that they alone can save the world from icy destruction.

Can they trust her? And can they overcome the terrible enemy which waits at the heart of the blizzards in the north?