The story features the adventures of two children, Kai and Kerttu, in Helsinki over 100 years ago. It is Kerttu’s birthday. Kai has made her a present himself, a glowing, snowy glass globe. When the two friends are together, the globe begins to glow. Kerttu understands that what Kai has given her is much more than just a gift.
Suddenly, the children’s life is changed. This change is precipitated by a mirror that their grandmother has bought on a trip to Lapland. The mirror is a magical mirror, stolen from the ice castle of the Snow Queen in the farthest North. The Snow Queen, enraged at the theft, has sent her gnomes to find the mirror and retrieve it. When they finally find the mirror at Kerttu’s home, they drop it in their haste, and it shatters into thousands of pieces – each carrying a bit of the freezing cold magic of the Snow Queen.
One of the shards flies into the eye of Kai as he sleeps, changing the way he sees the world around him. Another shard flies into the glass globe, extinguishing its glow. A third shard is found by a Lapland witch or seita, the half-sister of the Snow Queen. Desiring to recover all the pieces, the Snow Queen decides to kidnap Kai and have him reassemble the mirror at her ice castle.
Kerttu embarks on a long journey to find Kai at the ends of the world. She meets many different people on her travels. Finally, she ends up in the farthest North and talks to the Lapland seita. The seita and her horde of children invite Kerttu to meet the denizens of an underground world. Kerttu is able to conquer her fear of these strange creatures, as in her travels she has learned to understand different creatures. Kerttu discovers that everyone fears the Snow Queen, and they all combine forces to rescue Kai from the frozen castle.