A lost diary. A determined son. A torrid affair. A terrified man turns up at 221b insisting Irene and Joe take possession of a diary he is translating that was donated to the museum archives. The contents tell tales of murder, extortion, and other heinous crimes that this man wants no part of. Eager to seek out the author of this diary, the pair takes the book of his hands. However, when a man from France finds them, claiming the diary belongs to his father, he asks the pair to find the person who donated the diary to the museum in the first place. Irene and Joe have no choice but to help, as he hails from a very dangerous family that could threaten the safety of not only Irene and Joe, but everyone who calls 221 Baker Street a safe place.