An urban apartment building is the setting for this lively “who’s out there?” mystery that involves three kids, two families, two baby-sitters, two dogs, one pet rat, and a mistaken janitor. Lizzy lives on the top floor of the building with her parents and her dogs, Wag and Wave. Rowan and Ryan, new to the building, live on the bottom floor with their mother, grandmother, and a new pet rat. On a day when Lizzy’s baby-sitter, Nelly, and Rowan and Ryan’s baby-sitter, Nick, are out running separate errands, the janitor knocks on each of the two apartment doors, announcing that she (or is it he?) has come to fix a clogged sink. In each apartment, the children refuse to open the door. When the two babysitters meet and bring the kids together, more details of the janitor mystery unfold. There is plenty of opportunity in this detail-rich pairing of text and illustrations for readers and listeners to follow the action and figure out most of the clues as to who was knocking on the doors and why. For those who need a little assistance, all is finally revealed in the fourth and final chapter. (Ages 6-9)