Elliot has a new baby sister. He also has a room of his own now, where he can build castles out of blocks, as big as he likes.
But there is one thing that worries Elliot. In his new room, he can't fall asleep as easily as he did when he slept closer to Mom and Dad. On top of that, a monster is nesting under his bed. It growls every single night, even though no one believes Elliot.
Finally, with Dad's help, Elliot learns the cause of the strange noise, and he can sleep peacefully again.