It is 1944 and thirteen year-old Tolly Oldknow is sent to spend Christmas with his grandmother, whilst his mother searches for news of his father in wartime London. In the old house, he becomes a witness to events during the Napoleonic wars and finds himself slowly drawn into participating in the drama. Invisible to most people in the past, he begins to unravel the mystery which has bewildered his family for two centuries. The solution to the puzzle leads him into his greatest adventure yet.