Her life is changed when a quiet girl, Unn, moves to the village to live with her aunt after the death of her unmarried mother.

Unn does not want to feel embarrassed when meeting Siss the next day, so she decides to skip school and instead goes to see the ice castle that has been created by a nearby waterfall.

Ice castles are normal in cold winters, when the water freezes into huge structures around waterfalls.

Therefore, Siss takes upon herself the role Unn had: standing alone in the school yard refusing to play or speak.

The film focuses slightly more on Unn's secret feelings than the novel, but otherwise it's very true to the book, with the same slow snow-laden pace.