''Come round here!' called David. 'We've found something.'
'I'm showing them.' Andrea pushed him aside. 'It's my cave!' She lowered herself into the hole. Her head disappeared, and a moment later they heard her voice, faint and slightly hollow. 'Come on!'
A network of tunnels leading under the park, a secret exit to an abandoned mansion, a hidden box of documents...all very mysterious, but it's just a game, right?
When shadowy figures start watching every move they make, Kitty, David, Andrea and Martin know they've stumbled onto more than a forgotten piece of history. They need to find all the answers fast, before someone beats them to it.' (From the publisher's website.)