Philippa Watson, a good-natured yet troubled seventeen-year-old, has just moved to Washington, DC. She's lonely until she meets Judy Peabody, a brilliant and tempestuous classmate. The girls become unlikely friends and fashion themselves as intellectuals, drawing the notice of Christine Martins, their dazzling English teacher, who enthralls them with her passion for literature and her love of noirish detective fiction. When Philippa returns a novel Miss Martins has lent her, she interrupts a man grappling with her in the shadows. Frightened, Philippa flees, unsure who the man is or what she's seen. Days later, her teacher returns to school altered: a dark shell of herself. On the heels of her teacher's transformation, a classmate is found dead in the Anacostia River—murdered—the body stripped and defiled with a mysterious inscription. As the girls follow the clues and wrestle with newfound feelings toward each other, they suspect that the killer is closer to their circle than they imagined—and that the greatest threat they face may not be lurking in the halls at school, or in the city streets, but creeping out from a murderous impulse of their own.