After finding an old smugglers' tunnel while exploring, the boys fall into it and are unable to climb out. When they follow the length of the tunnel they discover stairs which lead to Craggy Tops' cellar. After Allison is befriended by the mysterious stranger, Bill, he offers to take the children out for a boat trip. But they don't trust him and set about trying to discover his true motives. (thetvdb.com)