Nathan Napp cannot sleep. He is leaning on his window ledge in the darkness of Christmas night, looking out over his father's model, garden railway. While keeping an eye out for any sign of Father Christmas's sleigh, he has no idea that he is about to be shrunk to three centimetres tall. Isabella, one of Father Christmas's five hundred elfin helpers, has a problem; her magic has gone terribly wrong, and Nathan soon finds himself tagging along in a model railway world that has come to life by Isabella's magic. Together they have to find a way back to reality before Nathan's parents wake to find him gone. The magic teases them, hindering their escape. Only someone else's magic can save them, but who?