"The Duchess Assignment" was the twenty-first episode of Thunderbirds season 1 and first aired on the 17th February 1966. It was the 23rd episode produced. The Duchess of Royston, an old acquaintance of Lady Penelope, has fallen on hard times through an unlucky spell gambling. A pair of gangsters become aware of her sole remaining asset, a precious painting known as Portrait of a Gazelle, and her life is endangered when the two men kidnap the duchess and attempt to steal her painting.