Dr Quimper is described as a "tall genial man, with a casual offhand cynical manner that his patients found very stimulating". He is forty-four years old. He drives a battered Austin car.
At Christmas-time, Dr Quimper treated Luther Crackenthorpe for a gastric attack. He was puzzled, because there were certain indications that were more consistent with arsenic poisoning than with gastro-enteritis. However, Luther said that he had had similar attacks before Dr Quimper started attending him, and they always happened when there was a lot of rich food. Dr Quimper wrote to Dr Morris, who had attended Luther before him, to find out more about these gastric attacks. Dr Morris had treated Luther for gastric attacks, and felt that Dr Quimper was being foolish.
After the discovery of a woman's body in the Long Barn at Rutherford Hall, Dr Quimper looks at the body. He says that the woman was not one of his patients, and that he does not remember ever seeing her about in Brackhampton.
Emma tells him about a letter she had received just before Christmas, from a woman claiming to be the wife of Edmund Crackenthorpe, Emma's brother who died during the war. Edmund had written to his family about a French girl named Martine, whom he intended to marry, but he was killed shortly after, and they had not heard from the girl until the arrival of this letter. Emma is not sure whether to tell the police about the letter or not, as she thinks it possible that the dead woman found in the Long Barn could be Martine, but her brothers all feel that it is unnecessary to tell the police. Dr Quimper advises her to tell the police, as she will go on worrying if she does not.
Later in the novel, Lucy Eyelesbarrow calls Dr Quimper when the Crackenthorpe family is taken ill. He attends to the family, and takes samples of the food they had eaten for dinner, to be tested. He also tells Lucy to look after two people in particular, Emma, who means a lot to him, and Luther.
Dr Quimper does a rough test on the curry that the Crackenthorpe family had eaten, and finds evidence of arsenic. He reports this to Inspector Bacon, and gives him the rest of the sample of curry, for the police to analyse. He says that it is his opinion that no one had consumed enough arsenic to cause death, and the idea was probably to put enough arsenic in to cause symptoms of food poisoning, and then one person would take a turn for the worse and die, having been given a second dose. While he is still there with Inspector Bacon, a call comes from Rutherford Hall, telling Dr Quimper that Alfred Crackenthorpe had suffered a serious relapse, and had died.
Harold Crackenthorpe later receives a parcel at his home in London. It contains a tablet box, with a small piece of paper saying that it has been sent by the request of Dr Quimper. Harold takes the tablets, and is dead by the next morning. Dr Quimper had earlier told Harold that he would not need to take the tabets anymore, and says that he did not request for the parcel to be sent.
At the end of the novel, Dr Quimper comes to tea at Rutherford Hall. It is his birthday, and they have made a cake for him. Miss Marple and Elspeth McGillicuddy are also there. Miss Marple gets a fishbone caught in her throat, and Dr Quimper attends to her. This allows Elspeth to see him from behind, holding Miss Marple's throat, and she recognises him as the man she saw strangling a woman on a train.
It is revealed that this was part of a plan by Miss Marple. She tells Dr Quimper that Elspeth saw him strangling the woman, and that she recognises him. Dr Quimper lunges at Miss Marple, but is stopped by Cedric Crackenthorpe and Bryan Eastley.
It is revealed that the woman who was strangled was Dr Quimper's wife. He had seen a chance of marrying a rich wife, Emma, but he was already married. His wife would not agree to a divorce, even though they had been separated for years. Miss Marple thought that this fit very well with what she had heard about Anna Stravinska, who had mentioned an English husband, and who was a devout Catholic.
Dr Quimper had earlier written the letter to Emma, which purported to be from Martine. Then, when she told him about it, he encouraged her to tell the police about it, because he wanted the dead woman to be identified as Martine, and for suspicion to fall on the Crackenthorpe family. Miss Marple also thought that he might have heard that the police were making inquiries about Anna, and so he arranged to have a postcard come from her, from Jamaica.
Miss Marple explains that Dr Quimper arranged to meet his wife in London, took her on the train, and strangled her. He pushed her body out of the train so that it pitched down the embankment, and returned later to hide it in the sarcophagus in the Long Barn at Rutherford Hall.
She also explains that he poisoned the Crackenthorpe family by putting arsenic into the cocktail jug. There had not been arsenic in the curry, but he had added it later, after taking a sample. It was then easy for him to poison Alfred, and send the tablets to Harold.
In MGM's 1961 adaptation Murder, She Said, Quimper is given the first name "Paul". The part of Quimper is played by Arthur Kennedy. How he is caught at the end is slightly different from the original. Miss Marple asks Alexander to show the murdered woman's power compact (which played music when opened) to the family. He tells them Miss Marple has it and lent it to him. Quimper sees it and fears that Miss Marple knows the background to it and suspects him. He approaches Miss Marple in her room and there Miss Marple reveals what she has surmised: that the murdered woman was his wife (she is not named in this show) and that she had refused to grant him a divorce to marry Emma. He had given the power compact as a gift to the murdered woman and now he feared it would trace back to him. Quimper admits all this as well as the other killings and then proceeds to try to kill Miss Marple by injecting something lethal into her but Inspector Craddock and others were hiding in the room and burst out to arrest him.
In BBC's 1987 adaptation of 4.50 from Paddington, Quimper is given the first name "John". Here his portrayal is fairly faithful to the original. His string of murders is motivated purely by money and Miss Marple thought that he would eventually plan to kill everyone else so that Emma would get the entire inheritance. It was even possible that he would then later kill Emma to get all the money for himself. Miss Marple thought that he might have deliberately refrained from diagnosing Alfred's terminal illness early on so that he would find out about it only after it was too late to seek effective treatment. She also attributed Luther's ill health to Quimper's bad advice. At the end, Miss Marple told Luther that he would be much better now that Quimper was not around to tell him how ill he was and she advised Luther to do all the things Quimper had advised him not to do. After being denounced by Miss Marple, Quimper attempted to escape but was confronted by Bryan Eastley before he could get into his car. Quimper drew a gun but Bryan advanced on him slowly, daring him to shoot and finally disarmed him. Quimper had some redeeming qualities which Emma pointed out early in the show. He was a firm supporter of the NHS, and refused to take on private patients because he believed in affordable and universal healthcare. The part of Quimper in this adaptation is played by Andrew Burt.
In NHK's anime adaptation 4:50 from Paddington, the portrayal of Quimper is close to that in the original novel. The voice actor behind the character is not credited.
In ITV's 2004 adaptation of 4.50 from Paddington, Quimper is given the first name "David". He is portrayed slightly more sympathetically than in the original novel. Here Miss Marple tells Emma at the end that he committed the murders purely out of his love for her. In this adaptation, Emma, because of the unusual conditions of the will, would not get any money after Luther died. Quimper killed his wife Anna Stravinska because she was a devout catholic and refused to grant him a divorce so that he could marry Emma. After this, Quimper attempted to distract the investigation by planting a fake sales receipt to "Martine" in the grounds for police to find. Alfred saw this and started to blackmail him and this led to Quimper having to silence him. The part of Quimper is played by Griff Rhys Jones.