Chad Wick is thirteen years old and knows the rules of walking home through Fitler Square: eyes forward, headphones in, mind your business. But when he glances down through a cellar grate on Cypress Street and sees a girl his age... bound, pale, watching him with a patience that doesn't belong to a child. He can't look away.

Her name is Mercy. She says she's a foster kid. She says the men who own the house won't let her leave. She says no one has ever tried to help her before.

Chad believes every word.

The Delancey family lives behind the black-painted brick of 2400 Cypress Street. Jack, a corporate fixer. Ryan, his chaotic younger brother. And Rina — a school counselor who opened her home and her heart to a girl who needed someone to care.

But something is wrong inside that house. Something that smells like bleach and old stone. Something that moves through the fog that rolls off the Schuylkill River at night. Something that has been in this neighborhood for far longer than anyone living.

In the old Philadelphia neighborhood called Devil's Pocket, the mill workers had a saying: the river provides. They never said what it costs.

Chad looked down. Now he can't stop falling.