The latest attack in a string of violent home invasions kills senior Elsie Englehart. Security officer Casey Holland is devastated. She is supposed to be watching over elderly bus riders in an affluent, high-risk area, but she's let Elsie down.

Determined to keep others safe, Casey escorts an elderly man home, but an armed intruder attacks them both. Hospitalized and angry, Casey struggles to regain control of her life, despite interference from family and colleagues-and the postponement of her long-awaited wedding.

Yet another home invasion compels Casey to take action, but at what cost to her health and her relationships? In Knock Knock, Debra Purdy Kong's fifth installment of the Casey Holland series, the risks have never been higher and the consequences more deadly.