Harry and his dinosaurs love birthday parties, so Harry's disappointed when big sister, Sam, says she's going to one - alone. Not only is it Mr. Oakley's birthday, but Harry finds out that it's the dinosaurs' one hundred millionth birthday too! Harry decides to throw a surprise birthday party and soon even Sam helps to make the party treats. Mum invites Mr. Oakley over and he is very surprised when he arrives; in fact, everyone agrees that it's the best birthday party ever!