After heroically saving his classmates, fifth-grader Sawyer's life as a dinosaur-human hybrid has gotten a lot easier until Sawyer, Sylvie, who is part Martian, and Elliot head to Mars in search of Sylvie's missing dad and discover that Mars is trying to kick Pluto out of the solar system.

With school out Oona and her brother Bonk are busy helping their father gather food, but Oona's pet Stegosaurus is something of a cave wrecker when left at home all day, so the children start Camp Woggle where all the local pets can amuse themselves during the day--but Mauro's pet T-Rex is a problem because he has a tendency want to eat the other campers.

Oona finds a giant egg and brings it back to West Wog to care for it, and see what hatches--but others in the cave want to claim her egg, particularly the nasty kid Bruce Brute, and when the baby stegosaurus hatches it will have to choose who it wants to live with.

"Now that Oona's dino-baby has hatched from its egg, things have gotten even wackier! Stacy the enormous stegosaurus, can't wait to explore her new world. And Oona can't wait to show her off. But not everyone's ready for this big bundle of joy whose having the time of her life romping and chomping through West Woggle. With Bonk, Bruce Brute, Erma, and even the mean witch, Old Brouhaha, on the case, this loveable babe finds her right place in the hearts...

When Tom and Jamie enter the Dino World this time, something is different. The mountains are volcanoes and the weather seems strange. They've gone to the Jurassic period, and there's a whole new world to explore.