- "Oh Penelope, dear. I'm a desperate woman. All I've got left is my painting."
— Deborah, Duchess of Royston
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
International Rescue Equipment Used
Non-International Rescue Equipment Used
- 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 about 100, far more than any other episode.
- This episode marks the third, and final, major appearance of the Mole. It also marks the first and only appearance of the DOMO, which is later revamped as the Excavator in Martian Invasion.
- This is the only episode where we see John Tracy with a frowned expressions.
- This is also the only episode with a female character voiced by a male character.
- When the 2nd croupier says, "Dix-huit rouge, 18 red", he is voiced by Peter Dyneley instead of Matt Zimmerman.
- When the WAF Wombat lands on the runway, the jetex attached on the underside falls off.
- 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; Il quadro della duchessa
- Dutch: Opdracht voor een hertogin (TV 1967); De opdracht van de hertogin (TV 1992); De ontvoering (VHS); Opdracht van de hertogin (VHS, DVD)
- Portuguese: O Acordo da Duquesa
- Japanese: 公爵夫人の危機