Tess is a Switcher, and Switchers are always alone - they have to be in order to protect their secret. No one would ever belive that a girl could become any animal that she chose. But Tess can.

Soon Tess realizes that the stranger, Kevin, is also a Switcher. He has been called by the animal world to stop the Northern Hemisphere from being destroyed by a mysterious, creeping iciness. And Kevin needs Tess's help to do it. Taking on and discarding one animal form after another, the two Switchers battle the icy evil.