A big brother is big trouble for Gregory. Mark is twice Gregory's age and also twice his size. Mark hogs the TV remote, grabs the best snacks first, and can shoot a basket without help. He can even squash Gregory with one hand. That really stinks! But one day, when Gregory sinks his teeth into a juicy steak, something remarkable happens -- he disappears. And in his place stands the towering dinosaur, G-Rex. Watch out, everybody, especially if you're someone named Mark.