Retired archaeologist and crossword setter Judith Potts believes that a brutal murder has taken place in the sleepy town of Marlow; when the police refuse to believe her story, she kicks off a private investigation with two unlikely friends.

Sisters September and July share an intense bond, speaking in a secret language and keeping the outside world at a distance. After September is suspended from school, their mother brings them to a remote house on the Irish coast—where July begins to resist her sister’s increasingly sadistic control.