Josh and Danny are on holiday with their parents in Cornwall-so the last person they expect to see is mad scientist, and next door neighbour, Petty Potts! So when the twins see her on the beach - holding a turtle under each arm-they can't help but wonder what she's up to. The thing is, the boys have got other things on their mind. They've found another mystery marble clue, and need to get out to the ruined fort on the rocky island. But they take too long and the tide comes in, trapping them. Luckily, they have a bottle filled with SWITCH spray and turn themselves into leatherback turtles! Their swim back to mainland is filled with danger . . . Will Danny be able to save Josh from the fishing net before he SWITCHes back in to a boy?