It's 1746 and Mary is desperately and dangerously single. Enter Lord Cranstoun, a recently disgraced naval officer who meets Mary at a garden party. But as the patterns are cut for the wedding dress of her dreams, she discovers Cranstoun has a wife back home in Scotland. The servants toss invitations into the fire and Mary, forbidden from seeing her lover, receives a love potion from him in the post. Purportedly to amend her family's disapproval of Cranstoun, the dose Mary delivers into her father's porridge kills him. It was arsenic. The court trial of the century ensues.