Dave and his friends get lost on a school orienteering exercise thanks to Robert Robot, who is unable to read the map - snatched by a flock of giant tiger pigeons and deposited into their equally giant nest, Dave must retrieve the map from Robert and get them all back to school.

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.