The Famous Five are almost relieved when Aunt Fanny's automobile breaks down, leaving them stranded in a small town while on their way to a party they don't want to go to. They kill time by visiting the local natural history museum, where recently discovered dinosaur bones are being shown. They meet Marty, who claims that his long-dead father discovered a whole dinosaur skeleton and created a map of its location. Sadly, the map has been taken. George, Julian, Dick, Anne, and Timmy decide to take a guided trip through the Valley of Dinosaurs since they can't let a good mystery go unsolved. However, when they think that the thief is one of the crew, their fact-finding trip quickly becomes a race to find the dinosaur.