Dave and his friends embark on a school trip along a canal where they awaken an ancient monster called Big Lippy, who pushes them over a waterfall and sends them back in time to the Jurassic era.

Dave is warned by local fortune teller Madam Donald that walking under a ladder will bring bad luck - he walks under one anyway, and soon a meteor is heading for Grimsby. It is up to Dave and his friend Robert Robot to catch it before everyone is squished.