Three children stumble upon an extraordinary library in an old house. Armed with magical books, and accompanied by an enchanting deerhound, they leave through a door that takes them to a world in the past. Beyond the door, a kingdom is in crisis.

The Great Sword of State has been stolen and destroyed. The Queen reveals an ancient prophecy that says only children can replace the Sword and she sends them on a journey that takes them beyond the world's end.

The children must unlock the secrets in their books as they face storms, fight traitors and tackle dragons.

Without knowing who they can trust, can they succeed?