"The Problem of Thor Bridge" is a Sherlock Holmes short story by Sir Arthur Conan Doyle first published in 1922 in The Strand Magazine and later collected in The Casebook of Sherlock Holmes which was first published in 1927.
When Maria Gibson is shot through the head while standing on the Thor Bridge on her estate, the family's governess, Grace Dunbar, is arrested for murder. Not only does she admit meeting her mistress on the bridge, the police find the murder weapon in her room. She vehemently denies killing the woman and Senator J. Neil Gibson, the dead woman's husband, believes her. He hires Sherlock Holmes to get to the bottom of it all. The Senator admits that he and Miss Dunbar have affections for one another but states that neither of them had anything to do with the murder. Dr Watson is convinced that the Senator himself is the culprit, but Holmes comes to an entirely different conclusion.
Neil Gibson, the Gold King and former Senator for "some Western state", approaches Holmes to investigate the murder of his wife Maria in order to clear his children's governess, Grace Dunbar, of the crime. It soon emerges that Mr Gibson's marriage had been unhappy and he treated his wife very badly. He had fallen in love with her when he met her in Brazil, but soon realised they had nothing in common. He became attracted to Miss Dunbar; since he could not marry her, he had attempted to please her in other ways, such as trying to help people less fortunate than himself.
Maria Gibson was found lying in a pool of blood on Thor Bridge with a bullet through the head and note from the governess, agreeing to a meeting at that location, in her hand. A recently discharged revolver with one shot fired is found in Miss Dunbar's wardrobe. Holmes agrees to look at the situation in spite of the damning evidence.
From the outset, Holmes observes some rather odd things about the case. How could Miss Dunbar so coolly and rationally have planned and carried out the murder and then carelessly tossed the murder weapon into her wardrobe? What was the strange chip on the underside of the bridge's stone balustrade? Why was Mrs Gibson clutching the note from Miss Dunbar when she died? If the murder weapon was one of a matched pair of pistols, why couldn't the other one be found in Mr Gibson's collection?
Holmes uses his powers of deduction to solve the crime, and demonstrates, using Watson's revolver, how it was perpetrated; Mrs Gibson, outraged and jealous of Miss Dunbar's relationship with her husband, resolved to end her own life and frame her rival for the crime. After arranging a meeting with Miss Dunbar, requesting her to leave her response in a note, Mrs Gibson tied a rock on a piece of string to the end of a revolver, and shot herself, the rock pulling the revolver over the side of the bridge; the revolver found in Miss Dunbar's wardrobe was the other pistol of the pair, which had been fired off in the woods earlier, and the chip in the bridge was caused by the pistol hitting the stonework as it was pulled off by the rock. Holmes's reconstruction reproduces the damage to the balustrade of the bridge. He asks the police to drag the lake for the revolvers of Watson and Gibson.
The story is notable within the Sherlock Holmes canon for the initial reference to a tin dispatchbox, located within the vaults of the Cox and Co. Bank at Charing Cross in London, where Dr Watson kept the papers concerning some of Holmes' unsolved or unfinished cases. According to Watson: "Among these unfinished tales is that of Mr James Phillimore, who, stepping back into his own house to get his umbrella, was never more seen in this world". The unknown fate of Phillimore has been a subject for other stories, including: The Adventure of the Highgate Miracle by Adrian Conan Doyle and John Dickson Carr; "The Enigma of the Warwickshire Vortex" by F. Gwynplaine MacIntyre; one episode of the Italian comic book series Storie di Altrove (a spin-off from the more famous Martin Mystère); and Bert Coules's BBC Radio adaptation The Singular Inheritance of Miss Gloria Wilson from The Further Adventures of Sherlock Holmes. Also mentioned is the case of Isadora Persano, "who was found stark staring mad with a match box in front of him which contained a remarkable worm said to be unknown to science".
- "Thor Bridge", an episode of the 1960s BBC television adaptations, from the programme's second series. Shot in colour, first aired in November 1968. Starring Peter Cushing and Nigel Stock. Unfortunately, this is one of the second series' missing episodes.
- Billy Wilder's 1970 film The Private Life of Sherlock Holmes (written by Wilder and I.A.L. Diamond) uses for its opening title sequence a montage of characteristic memorabilia (magnifying glass, calabash pipe, syringe) removed one by one from this tin dispatchbox at Cox and Co and held before the camera as music by Miklos Rozsa, including a plaintive violin solo, plays underneath. The voice-over narration by Dr Watson (played by Colin Blakely) generally parallels the opening sentence of "The Problem of Thor Bridge".
- "The Problem of Thor Bridge", the 1991 episode of the ITV / Granada Television series, starring Jeremy Brett and Edward Hardwicke.
- BBC Radio 4 adapted the short story under the same name in 1994, as part of a long-term series of BBC radio adaptations of the whole Sherlock Holmes Canon, directed by playwright and radio drama director Bert Coules. The radio adaptation starred Clive Merrison and Michael Williams.
- "You Do It To Yourself", the ninth episode of the first series of Elementary, borrows elements from this story.