The Duke's Folly was the first short story in the Short Trips anthology Short Trips: Seven Deadly Sins. It was written by Gareth Wigmore. It featured the First Doctor, Ian Chesterton, Barbara Wright and Susan Foreman.
Ian and Susan are in the American west, and Ian is fighting a duel with Slim, who insulted Susan. Slim is very nervous and misses his shot, then he flinches when Ian tries to shoot past him and receives a hit to his ear.
In England, the Doctor and Barbara are posing as Earl Foreman and his daughter. The Doctor is the guardian of David Warblington, the young Duke of Chilgrove. He wants the young man to be more like his father, who had saved the Doctor and Susan's lives in India, but David is more concerned that his American girlfriend, Joy St Peter, saw him fall off his horse during a hunt.
Ian and Susan, who are on a mission to find out more about Joy's family in America, make plans to leave Arizona and continue their quest.
The next morning at breakfast, Barbara informs the Doctor that David was sent down from Cambridge University and lost money betting. She wonders why the Doctor is spending so much effort on David, and reminds him that while she is familiar with David's father's impact on English history, she has no knowledge of David. The Doctor informs her that David's father saved his and Susan's lives and in return he will do whatever he can to help David.
In San Francisco, Ian charts a boat for Holland and recalls the adventures he and Susan have had in America. Their travels have been so difficult he is glad to be leaving, but as he wanders the streets, he finds a clothing shop owned by Joy's father. He contemplates continuing his investigation, but instead heads back to the hotel where Susan is waiting.
Back in England, Warblington Place, the family home in Richmond, is on fire. Barbara and the Doctor try to get close, wanting to make sure David is all right. He soon joins them and tells them the servants are all accounted for. Then he admits that he forgot to put out his cigar, causing the fire.
After Ian and Susan meet up again with the Doctor and Barbara in England, David and Joy marry. Susan is not allowed to intend in case someone in attendance recognises her from decades ago in India. David tells the Doctor that he and Joy plan to move to Oklahoma, and David confides in Ian that the Doctor took more interest in him than his father ever did.
In the TARDIS, when Ian tries to mention this to the Doctor, the Doctor refuses to hear anything negative about his old friend. He intends to take them to London, 1865, to show them what kind of father his friend was, but the TARDIS has instead taken them to a land covered in grasshoppers and having four moons.
- First Doctor
- Susan Foreman
- Barbara Wright
- Ian Chesterton
- The mayor
- David Warblington
- Joy St Peter
- The Doctor met David's father in India during the Indian Mutiny.
- Ian and Susan visit several American states, including Arizona, Idaho and California, and several American cities, including Boston, Omaha and Santa Fe.
- The Doctor uses the alias Earl Foreman.
- The stories in Short Trips: Seven Deadly Sins are based on the Christian concept of seven deadly sins. This story's theme is sloth.