Like most kids with younger siblings, Lucy loves her sister Antonia, but is all too frequently annoyed with her as well. The two spend most of their time together since their mother works so hard, but without friends Lucy can't help but feel lonely anyway. She's always tried to ignore the bullying and teasing at school about her family being poor--it's always best to keep her head down and do nothing.

When the girls find an old, muddy doll head in the river, Antonia claims it as her newest treasure. At night Lucy hears her talking to Hush-a-Bye--and does she hear the doll talking back? Soon, Hush-a-Bye seems to be protecting Antonia by making bad things happen to others, and it isn't long before Lucy asks for its help against her tormentors, too. Slowly Hush-a-Bye's influence forms a wedge between the sisters, and Antonia's dependence on it becomes frightening. The doll has a mind of its own, and soon it will have Antonia's as well. Can Lucy solve the mystery of Hush-a-Bye to stop its evil plans?