Jack and Annie are siblings who live in the small American town of Frog Creek. The two of them could not be more different, as Jack reads all the books he can get his hands on, while Annie charges headfirst into adventure, even if it means getting up close and personal with a muddy frog. When the two kids happen upon a treehouse full of books in the woods, they are shocked when Jack's offhanded comment about wanting to visit the land of dinosaurs—depicted in one of the books they find—ends up transporting both them and the treehouse millions of years into the past!

While Annie is excited to explore their new surroundings, Jack is more concerned about getting home in one piece. However, when he finds a strange medallion that clearly was made by other humans, Jack's find soon kicks off an adventure that will take the siblings across ages and continents.