The small Caravaggio which hangs in the chapel of Doon Abbey in Kildare attracts visitors to the abbey from far and wide. One night, however, the painting disappears. The Sister Superior is unwilling to cooperate with the police, as this would compromise the abbey's silent-order ethos. Alice Dunwoody, a novitiate at the abbey who heard strange sounds on the night of the theft, persuades Sister Mercy Superior to allow her to investigate, with the help of the abbey's computer-savvy librarian, Sister Mary Magdalene. As the nuns try to track down the painting, the list of possible suspects and the body count multiplies.