It is the summer of 1810. The Lyon family of London is eagerly preparing for the wedding of their eldest daughter Hannah. But then, Mr. Samuel Lyon, a member of London's Jewish elite, suffers a crushing blow. In the blink of an eye, Mr. Lyon's entire fortune is lost. But help soon arrives from an unexpected source: Mr. Ezra Melamed, wealthy widower and benefactor of London's Jewish community. With only a key, a button, and a few cryptic words from a Chassidic Rebbe to guide him, Mr. Melamed goes beyond London's fashionable streets to search for clues in the darkest places.