One balmy June evening in 1881, Phoebe Stanbury stands before the guests at her engagement party: this is her moment, when she will join the renowned Raycraft family and ascend to polite society. As she takes her fiance’s hand, a stranger holding a knife steps forward and ends the poor girl’s life. Amid the chaos, he turns to her aristocratic groom and mouths: ‘I promised I would save you’. The following morning, just a few miles away, timid young legal clerk William Lamb meets a reclusive client. He finds the old man terrified and in desperate need of aid: William must keep safe a small casket of yellowing papers, and deliver an enigmatic message: ‘the finder knows’.