In the short story The Coming of Mr Quin, Derek Capel was the former owner of the country house named Royston. Mr Capel had comitted suicide ten years before the events in the story. The reason for his suicide was a mystery. Discussing the circumstances of the suicide and divining the reasons for it forms the main part of the plot of the story.
At the time of his death, Capel had five or six friends staying with him, including Tom Evesham and Sir Richard Conway. At dinner, he had announced to his guests that he was engaged to be married. He had not told anyone who the woman was but seemed happy about the prospect. He seemed like a man who had "staked heavily and won against well nigh overwhelming odds".
Capel had left the room to get the post, which was a cause for excitement as they had been snowed in for three days. He had opened the newspaper to see if there was any news, and had then gone upstairs with his letters. He shot himself a few minutes later. The natural solution was that he had received unexpected news in a letter, but his letters were all found unopened.
Capel had no troubles with money, so his friends thought that the cause of his suicide had something to do with a woman. They also thought that the woman to whom he was engaged was Majorie Dilke, as they were great friends, and had been together a lot.
Royston did not have a telephone, but the local constable happened to be in the kitchen at the time. He had come to bring back Rover, Capel's dog, which had strayed the day before.
Capel had known the Appletons, and had stayed with them the previous Spring, a week before Mr Appleton had died. He had spoken before about what an "old curmudgeon" Mr Appleton was, and how awful it must have been for Mrs Appleton to be married to him.
In the present, it is suggested that Capel had poisoned Mr Appleton's port with strychnine, but Mr Appleton had not died until about a week after Capel had left, because strychnine is not very soluble. Capel had later persuaded Mrs Appleton that he was innocent, and she consented to marry him. However, the engagement could not be announced immediately because her husband had just died.
It is further suggested that Capel had seen a paragraph in the newspaper saying that the Home Office had given permission to exhume the body of Mr Appleton. He had then looked out of his bedroom window, which gave on the drive, and had seen a policeman. He did not know that the policeman was coming to bring back his dog, and thought that he was coming about the Appleton case.
In The Love Detectives, Mr Satterthwaite tells Colonel Melrose that it is due to Harley Quin that the mystery was solved.