Cealie Gunther, an independent woman of a certain age, finds her granddaughter, Kat, in tears on a surprise visit to her widowed son in a Chicago suburb. Kat, a high school senior with excellent grades, is so upset after the death of a school custodian (the police, who at first ruled the custodian's balcony fall an accident, now suspect Kat's Spanish teacher of murder) that she insists on staying home and missing final exams and graduation. Cealie goes to investigate.