Nigella Mason, Marchioness of Loudwater, is the ex-fiancé of Mycroft Holmes and a British peeress. Sherlock Holmes, suspected she was only interested in Mycroft for his wealth and successfully seduced her to prove it.
|Nigella retains the title of Marchioness of Loudwater, following her divorce from The Most Honourable Lord Loudwater, Robert Suffolk, The Marquess of Loudwater. After Mycroft arranges a meeting at Diogenes for her, Sherlock and Joan Watson, she explains that her divorce from the Marquess came with a champion stud race horse, Silver Blaze. The stable was broken into and the stable head, Mason, (her new lover) was killed and intent to poison Silver Blaze discovered. She'd like to find the culprit. She inadvertently reveals that Mycroft has leukemia. Sherlock wants nothing to do with her and leaves. Mycroft is able to convince Sherlock to help and Nigella found a bone marrow donor for him and, that if Sherlock helps, it'll prove that Sherlock sleeping with her is a thing of the past. Sherlock finds that an assassin for the Robles drug cartel is behind the murder.
|After Nigella survives a near miss from the assassin, she denies any ties to the cartel. Sherlock discovers the stable that the cartel head, Joaquin Aguilar, owns and that Silver Blaze studded one of his mares and produced a foal. After seeing the foal, Holmes deduces that Silver Blaze wasn't the stud. They confront Nigella who admits that Silver Blaze died and she passed off his brother for stud. Sherlock deduces that Aguilar knows the fraud and is seeking revenge. A trap is set for the assassin who is arrested. Sherlock settles the matter with Aguilar. Nigella meets Sherlock and Mycroft at Diogenes and is instructed by Mycroft to repay all the victims of her scam and use the money that Aguilar had given her for the fake stud services to begin a more respectable start in life, with the repayments overseen by both Sherlock and Mycroft. ("The Marchioness")|