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.

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.