Tom Austen and his sister Liz shiver with excitement when they arrive at Casa Loma, a castle right in the heart of downtown Toronto - one with a mystery to solve and a lost treasure to find. Sir Nigel Brampton, the millionaire owner of Casa Loma, has disappeared, and the police believe he has been kidnapped by thieves who are after the priceless diamonds that lie hidden somewhere in the castle.

Who is behind Sir Nigel's strange disappearance? Is it Smythe, the butler? Or Irene, the pretty maid Tom catches snooping in Sir Nigel's room? And what about the sinister blacksmith in the stables?

There are plenty of suspects to keep readers guessing right to the end of this fast-paced detective thriller.