When Henry Harman's father buys an old windmill in France, he and his little brother think it might be a nice adventure. But up in the roof of the windmill lives an old owl that the locals refer to as the Protector. But what is he protecting and why won't any of the builders go inside the building. When Henry does manage to get up into the roof, he discovers an ancient diary written by a boy 850 years earlier. The boy says he knows a secret, one so dangerous that he dare not write it down. As Henry and his brother decipher the code, things start to fall into place and they set out on an adventure of a lifetime. But can they get home again...?