Mr Satterthwaite attends a showing at an art gallery by a rising young artist called Frank Bristow. There he is stopped in his tracks by a painting called "The Dead Harlequin" which portrays a dead figure on a floor and the same figure looking in through an open window at his own corpse – the man portrayed is Mr Harley Quin and the room shown is the Terrace Room at Charnley, a house in the ownership of Lord Charnley which Satterthwaite has previously visited. He immediately buys the picture and meets the artist, whom he invites for dinner that night at his house.
The artist arrives as promised. Satterthwaite has set a place for Quin, fully expecting his friend to put in an appearance, and also a Colonel Monkton who was at Charnley the night fourteen years ago that the previous Lord Charnley committed suicide. The house has a ghostly history, with the spectre of Charles I walking headless on the terrace and a weeping lady with a silver ewer seen when there is a tragedy in the family. Several of the holders to the title met violent deaths but what made this latest death so strange is the circumstances in which it happened: the occasion was a fancy dress bell to celebrate the return from honeymoon of Lord Charnley and his new bride. Colonel Monkton was one of several people who stood at the top of a flight of stairs and saw Lord Charnley pass below. A woman called out to him but he walked on as if in a daze. He passed through the Terrace Room and into the Oak Parlour that leads off it. This latter room had several legends attached to it including Charles I hiding in a priest hole there, duels taking place with the bullet holes still in the wall and a strange stain on the floor which also reappeared even when the wood was replaced. The people on the stairs heard the door lock behind him and then a shot. They couldn't get into the oak Parlour using either a second locked door from another room or through the shuttered window and so they broke the door down and found the body with curiously little blood coming from it. No one benefited from the death, not even the next in line to the title, Hugo Charnley, as Alix, Lord Charnley's widow, was revealed to be pregnant and when her boy was born later on, he automatically inherited.
Satterthwaite receives an unexpected guest – Aspasia Glen, the celebrated dramatist. She was at the gallery and was also taken with "The Dead Harlequin" and wants to buy it. Satterthwaite dislikes the lady's false coquettish manner and is both relieved and surprised when Alix Charnley telephones him part way through his conversation also wanting to buy the picture. Recognising that events are coming to a head as they always do in matters involving Mr Quin, Satterthwaite asks Alix Charnley to come round to the house immediately. He escorts Miss Glen to where his other guests are and is not surprised to see that Mr Quin has arrived. When Alix also joins them, she is introduced to the others recognising Miss Glen, presumably from one of her stage performances.
They all start to relive the night and now, with the space of fourteen years having passed, Alix feels able to reveal that the reason for the suicide was a letter Lord Charnley received from the invited governess telling him that she was pregnant by him – just a month after his marriage to Alix. Monkton thinks that all is explained but Satterthwaite wants to know why Bristow's picture portrays the dead figure in the Terrace Room and not in the adjoining Oak Parlour. It is almost as if the death occurred there and the body was put in the Oak Parlour afterwards but as Monkton says, they saw Lord Charnley walk into the Oak Parlour. Satterthwaite points out though that they saw a figure in fancy dress who could have been anyone. The only person who called him "Lord Charnley" was the same person who was allegedly pregnant by him. A shot then fired in the locked room could have been accounted for by another bullet hole in the wall to go with the ones created by past duels and there is a priest hole to hide the person just seen entering the room. Any bloodstain on the floor of Terrace Room would have been covered by valuable red Bokhara rug which seems to have been placed there on that night only and the body was therefore dragged on the rug into the Oak Parlour. The stains on the rug could have been cleaned up by a lady with a ewer and if anyone sighted her, she would have been taken for the house's ghost.
Alix suddenly recognises Miss Glen as the woman who called out to Lord Charnley that night. The accused woman rushes out of Satterthwaite's house after confessing that she loved Hugo - who was the man seen in the fancy dress costume, Lord Charnley already being dead - and helped him with the murder but he abandoned her soon after and died the previous year. Alix is relieved. The letter to her husband was false to give a motive for suicide and she can now tell his son that his father has no stain on his reputation. After that picture's artist prepares to leave and discovers that Mr Quin has already left without being noticed.
The short story is a reworking of an unpublished stage play called Someone at the Window.