When Cassie was a little girl, her grandmother told her a fairy tale about her mother, who made a deal with the Polar Bear King and was swept away to the ends of the earth. Now that Cassie is older, she knows the story was a nice way of saying her mother had died. Cassie lives with her father at an Arctic research station, is determined to become a scientist, and has no time for make-believe.

Then, on her eighteenth birthday, Cassie comes face-to-face with a polar bear who speaks to her. He tells her that her mother is alive, imprisoned at the ends of the earth. And he can bring her back — if Cassie will agree to be his bride.