Andy Day travels back in time after promising to get some more salt for Hatty so she can finish her display of a fossilised footprint of an allosaurus, a massive meat-eating dinosaur that roamed the salt flats of North America 145 million years ago. Along the way, he befriends one of the creatures and gets caught up in chase along the salt flats as they dodge the giant legs and long necks of a herd of diplodocus