The solicitors firm of Merryweather, Linnell and Daventry, receives "a rather peculiar commission" from an American millionaire, Laurence P. Stewart, to purchase at auction some relics of Mary Queen of Scots; the following night, the treasures are stolen and a watchman is murdered. The next morning, Laurence Stewart's son contacts the firm to cancel the commission, explaining that his father was murdered the night before, and requesting that the solicitors send someone to assist. Daventry calls on his brother's friend Anthony Bathurst, and the two set off to unravel the double mystery.