Five on a Secret Trail by Enid Blyton

George and Timmy are camping alone when they meet a boy named Guy and discover an old ruined house nearby. Soon, the rest of the Five join, and strange noises and movements around the ruins suggest someone is hiding something. The children begin investigating and uncover a secret trail connected to an old theft. As they follow the clues, they navigate underground tunnels and outwit the villains. This mystery is full of teamwork, courage, and classic Enid Blyton intrigue.