When word gets out that Mary-Margaret O'Reilly may have been witness to a miracle, religious mania descends on the Church of the Sacred Heart in Battersea. The consequences will be profound, not only for Mary-Margaret but for others too: Father Diamond, the parish priest, who is in the midst of his own crisis of faith. Stella Morrison, adrift in her marriage and aching for her ten-year old son, away at boarding school. Alice Armitage, counting the days until her soldier son comes home from Afghanistan. Mary-Margaret's mother, Fidelma, imprisoned in a tower block with nothing but her thoughts for company. Mary-Margaret's quest to understand what she has seen grows increasingly desperate — but no one is prepared for the shocking outcome that ensues.