The heavy steel door clanged shut behind her with a finality that made Sister Claire’s heart stutter.

At twenty-four, she was the youngest novice in the convent of the Sisters of Divine Mercy, still adjusting to the weight of her black habit and the white wimple that framed her innocent face. Her superiors had chosen her for this assignment precisely because of her gentle nature and unwavering faith. “The prisoner has requested spiritual counsel,” Mother Superior had said. “You will go twice a week. Offer prayer, listen, and remind him of God’s infinite mercy—even for the damned.”

They hadn’t told her much else. Only that his name was Marcus Vale,...