Some doors keep things out.This one remembers who walked away.When Nathan moves into his new apartment, the bedroom door is just another cheap panel of painted wood.Until it knocks—softly, deliberately, and only when he's alone.At first, he blames the pipes. The neighbors. His nerves.But the knocks have a pattern …a rhythm he hasn't used since childhood …the same rhythm he used to knock on his parents' door the night his father died.