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|Portrayed by||Olivia d'Abo|
|First seen||The Marchioness|
|Last seen||The Marchioness|
|Significant other(s)||Mycroft Holmes (ex-fiancé)|
Nigella Mason, Marchioness of Loudwater, is the ex-fiancé of Mycroft Holmes and a British peeress.
Nigella Mason is Mycroft Holmes ex-fiancé and the Marchioness of Loudwater. Mycroft's brother, Sherlock Holmes, suspected she was only interested in Mycroft for his wealth, and so successfully seduced her to prove it. She retains the title of Marchioness of Loudwater, following her marriage to The Most Honourable Lord Loudwater, Robert Suffolk, The Marquess of Loudwater. She came out of her marriage to Lord Loudwater with her title of Marchioness of Loudwater and ownership of one of Lord Loudwater's horses, a champion stallion, Silver Blaze. 10 weeks after her divorce from Lord Loudwater, Silver Blaze died of heart failure, so she did with Silver Blaze's brother what she had intended to do with Silver Blaze. She made him a stud and herself a horse pimp. In this scam, of passing of Silver Blaze's brother as Silver Blaze himself she got involved with the Robles cartel, specifically a man by the name of Joaquim Aguilar and when they discovered that it was a scam, they almost killed her. With the help of Mycroft, Sherlock and Joan Watson she escaped unharmed. She was then instructed by Mycroft to repay all the victims of her scam and use the money the Joaquim Aguilar had given her to begin a 'more respectable start in life', with the repayments overseen by both Sherlock and Mycroft.
|Season two appearances|
|Step Nine||Solve for X||We Are Everyone||Poison Pen|
|Ancient History||An Unnatural Arrangement||The Marchioness||Blood Is Thicker|
|On the Line||Tremors||Internal Audit||The Diabolical Kind|
|All in the Family|