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.
- Main article: The Duchess Assignment/Storyline
- Main article: The Duchess Assignment/Transcript
- Jeff Tracy - Peter Dyneley
- Scott Tracy - Shane Rimmer
- Lady Penelope Creighton-Ward - Sylvia Anderson
- Virgil Tracy - David Holliday
- Alan Tracy - Matt Zimmerman
- Parker - David Graham
- Tin-Tin Kyrano - Christine Finn
- John Tracy - Ray Barrett
- Grandma Tracy - Christine Finn
- Captain Hanson - David Graham
- Fireflash Co-Pilot - Ray Barrett
- Deborah, Duchess of Royston - Ray Barrett
- Wilbur Dandridge III - David Graham
- Chandler - Peter Dyneley
- Brophy - John Tate
- Miss Godolphin (voice only, not seen on screen)- Sylvia Anderson
- Casino Owner - Peter Dyneley
- 1st Croupier - Peter Dyneley
- 2nd Croupier - Matt Zimmerman
- Hendricks - Matt Zimmerman
- Air Force Officer Percy - David Graham
- Percy's Friend - Ray Barrett
- New York Airport Customs Officer - David Graham
- Air Terrainean Hostess - Sylvia Anderson
- Grand Hotel Receptionist - Christine Finn
- Elevator Voice - David Graham
- Casino Croupiers
- Gamblers in the French Casino (duchess's first visit)
- Gamblers in the French Casino (duchess's second visit)
- Hotel Reception Guest
- Air Show Guests
- Art Gallery Visitors
- New York street scene
- New York Airport Arrivals
International Rescue Equipment UsedEdit
Non-International Rescue Equipment UsedEdit
- Art Gallery
- Country House
- Departure Lounge - London Airport
- French Casino
- Gazelle Automations
- The Grand Hotel
- The London Air Display
- London International Airport
- New York Central Airport
- Royston Castle
- Captain Hanson and the Fireflash co-pilot from Trapped in the Sky, return as the pilots of Fireflash.
- In the Australian half hour cliffhanger version, part one ends when Fireflash levels out into horizontal flight position.
- The Duchess of Royston was voiced by Ray Barrett, the same voice actor as John Tracy. This was because neither Sylvia or Christine Finn could provide the voice Gerry needed. However, Ray Barrett did a lot better than expected with voicing the Duchess.
- Virgil's painting of Alan from Move - And You're Dead can be seen hanging in the art gallery.
- At the end of the episode, Parker reveals that the Derby was won by Desperate Intruder, named after the episode of the same name.
- Although the name Braquasso is clearly an allusion to Pablo Picasso, the style of "Portrait Of A Gazelle" is much more similar to that of Salvador Dali.
- Including the silent (cameo) characters, The Duchess Assignment has a total cast of well over a hundred, far more than any other episode.
- When the WAF Wombat lands on the runway, the jetex attached on the underside falls off.
- When the 2nd croupier says, "Dix-huit rouge, 18 red", he is voiced by Peter Dyneley instead of Matt Zimmerman.
- As the Mole is leaving the pod the sign inside reads "Thunderbird 2 Pod 4" despite the fact the door was labelled as being Pod 3.
- French: La duchesse
- German: Das Gemälde der Herzogin
- Spanish: Asignación Duquesa; El rescate de la Duquesa (DVD)
- Italian: Una serata sfortunata
- Dutch: Opdracht voor een hertogin (TV 1967); De opdracht van de hertogin (TV 1992); De ontvoering (VHS); Opdracht van de hertogin (VHS, DVD)
- Japanese: 公爵夫人の危機