One day, Sully finds an activated door on his scare floor after the workday has ended. He finds no one in the room behind the door, but a little two-year-old girl follows him back into the monster world. Far from being scared, she calls him "Kitty" and delights in playing with him. Since monsters believe humans are lethally toxic, Sully tries repeatedly to return the girl to her room, but she keeps following him back, and Randall eventually deactivates and stores the door, leaving the girl stuck in the monster world. The girl's presence becomes public knowledge after Sully sneaks her into a restaurant to find Mike, so they hide her in Sully's home while the Child Detection Agency searches for her. Sully eventually names the girl "Boo", and he slowly bonds with her after realizing that she is not poisonous. He also discovers that her laughter produces even more energy than her screams.