Thomas's class is taking a trip to Devil's Island, and Thomas is bringing along his bionic best friend, McGrowl. Thomas and McGrowl can't wait to take part in all the fun nature activities. But the boy-and-dog dream team soon notice something fishy about the trip's leaders, Pop and Mamie Keeler. Could this pair possibly be the evil duo, back to cause more mayhem? And if so, will they use a dangerous volcano on the island to destroy Cedar Springs once and for all? It's up to Thomas and McGrowl to find out the truth about the Keelers and put a stop to their plans..before it's too late!