In the seaside village of Watchcombe, young Kate is determined to make the most of her last week of summer holiday. But when she discovers a mysterious painting entitled The Lord of Winter in a charity shop, it leads her on an adventure she never could have planned. Kate soon realises the old seascape, painted long ago by an eccentric local artist, is actually a puzzle. And with the help of some bizarre new acquaintances — including a museum curator's magical cat, a miserable neighbour, and a lonely boy — she plans on solving it.

And then, one morning Kate wakes up to a world changed forever. For the Lord of Winter is coming — and Kate has a very important decision to make.