In the novel One, Two, Buckle My Shoe, Sylvia Chapman was a resident of King Leopold Mansions in Battersea at unit 45. She came to the attention of the police after the disappearance of Mabelle Sainsbury Seale. The porter at King Leopold Mansions read about the disappearance on the newspapers and remembered that a lady by the name of Sainsbury Seale had come to the building looking for Mrs Chapman. He had shown her up to the flat. His wife also remembered a second visit by the woman. Unfortunately for the police, it was almost a month before the porter reported what he knew to the police.
Detective Sergeant Beddoes who responded to the report learnt that no one had seen Mrs Chapman for a while either. There was only a note on the door: “No milk. Tell Nellie I am called away.” Strangely, the porter had not seen Mrs Chapman leave nor had she asked for help e.g. with luggage. A search warrant was applied for, and a pass key obtained from the building manager. Inside the apartment, Beddoes found the body of a murdered woman who fitted the description of Mabelle Sainsbury Seale.
Police next approached another resident Mrs Merton, who the porter said knew Mrs Chapman best of all. According to Mrs Merton, they had played bridge a few times, and occasionally went shopping or to the movies together. She also knew that Sylvia's husband Albert Chapman was supposedly a commercial traveller working for some armaments firm and was seldom home. Mrs Merton then revealed that once, when watching a movie about spies, Mrs Chapman had, indiscreetly it seemed, revealed that her husband worked for the Secret Service and that the armaments firm job was only a blind.
Only a few other clues were found in the Mrs Chapman's apartment, significantly,in an address book was written the address of her dentist: the of Henry Morley of the practice at 58 Queen Charlotte Street. Morley had himself been murdered some time earlier.
More confusion was brought about when, at the inquest, Mr Leatheran, who had taken over the practice after the death of Morley testified that based on Morley's dental charts, the body of the dead woman in the flat was Mrs Chapman and not Mabelle Sainsbury Seale.
It is only towards the end of the story, during the denouement, that Poirot is able to piece together the different strands and reveal that Sylvia Chapman is actually an elaborate persona, one of many, adopted by Gerda Blunt in order to hide a bigamous marriage and later to commit several murders to protect that dark secret. In constructing the persona, Gerda and her accomplice Alistair Blunt had deliberately based it on an actual intelligent agent Albert Chapman in order to explain the fact that Mrs Chapman's husband was so seldom seen. As part of the deception, the conspirators had falsified the dental records kept by Mr Morley so Mr Leatheran, relying on them, would misidentify Mabelle Sainsbury Seale as Mrs Chapman. How this was done is a matter of conjecture. The conspirators had killed Morley and gained access of his office. Did they create a record for a totally non-existent patient Sylvia Chapman, or simply switch the records of two existing patients Sylvia Chapman and Mabelle Sainsbury Seale. In Chapter 7, the text records that Gladys Nevill, Morley's secretary "recollected Mrs Chapman". This suggests that Gerda Blunt did in fact visit and use Morley's services under her persona of Sylvia Chapman. So her record would exist and would merely need to be swopped with Mabelle's.