On any given day in London, all Charles Lenox, Victorian gentleman and armchair explorer, wants to do is relax in his private study with a cup of tea, a roaring fire and a good book. But when his lifelong friend Lady Jane asks for his help, Lenox cannot resist another chance to unravel a mystery, even if it means trudging through the snow to her townhouse next door. One of Jane's former servants, Prudence Smith, is dead - an apparent suicide. But Lenox suspects something far more sinister: murder, by a rare and deadly poison.