Zigzoo builds a train in the sap caverns to help Squirmtum with his work but gets carried away making it into a fun ride and doesn't build it properly. Not listening to Squirmtum's warnings has hair-raising consequences and Tom and Twigs become embroiled in what is literally a rollercoaster adventure! Tom can only save his friends, the sprites, the caverns and the day with Big World Magic help from the audience.