It's the first day of fourth-grade for twins Andy and Anna. Andy is a shy boy obsessed with dinosaurs and Anna is an adventurous girl willing to take risks. One day when they step off the school bus, Anna suggests taking a shortcut through a dark alley to get home.

As they navigate through the abandoned alley, they find a mysterious large egg tucked under a bush. They take it home and try to hatch it, careful not to tell their parents who are strict about pets. After building a makeshift nest in the attic, the egg hatches into a friendly baby velociraptor, whom they later name Tennyson, due to his infatuation with destroying tennis balls.

Now they must convince their parents to keep the little dinosaur, while overcoming bad grades and the school bully.