Mr Satterthwaite has been attending a newsworthy trial at the Old Bailey and on the final day hears the judge's summing up, the verdict of guilty and the sentence of death. He wanders across to the select and expensive Arlecchino restaurant in Soho where he finds Mr Quin sitting at a table, and tells him of the result of the trial. They begin to discuss the case, with Quin stating that he has not heard all of the evidence.
Lady Vivien Barnaby was the young wife of a far older husband, Sir George Barnaby, and trapped in a loveless marriage. She started to make advances to a local young gentleman farmer named Martin Wylde, seeing in him a means of escape from her life. Wylde did enter into an affair with Lady Barnaby but at the same time was involved in a relationship with the daughter of the local doctor called Sylvia Dale. Lady Barnaby started to get more hysterical with Wylde and on the morning of 13th Friday sent him a letter begging him to come to her house at Deering Hill that night at six o'clock when her husband would be out at a bridge game.
Although he at first denied it, Wylde did go to the house and his fingerprints were found in the room where, at 6.20pm, Lady Barnaby was killed with a single blast from a shotgun. The servants all heard the shot and rushed to the room. No one was there but their dead mistress. They panicked for several minutes then tried contacting the police but found that the phone was out of order. One of them went on foot and met Sir George returning from his game. All of the parties involved had alibis – Sir George left his game just before 6.30pm, Sylvia Dale was at the station seeing a friend off on the 6.28pm train and Sir George's secretary, Henry Thompson, was in London on business. Wylde admitted under questioning that he took his gun to Deering Hill but that he left it outside the door and forgot it when he left the house, in a high temper due to a scene he had had with Lady Barnaby. He claimed to have left the house before the time of the death but gave a reason for the time it took him to get home which no one seems to believe.
Quin enigmatically asks about a servant who gave evidence at the inquest but not at the trial and Satterthwaite tells him that she has gone to Canada. Quin's attitude to this fact prompts Satterthwaite down the line of wondering if he should go there and interview the missing housemaid. Satterthwaite tracks the maid, Louise Bullard, to Banff and goes on an ocean voyage to Canada where he finds her working in a hotel. She is an impressionable girl who speaks of seeing the shape of a giant hand in the sky caused by the smoke of a passing train at the very time she heard the shot. She does however tell Satterthwaite that Henry Thompson suggested the post in Canada to her, the job paying an extremely large wage, although she had to leave quickly to take it.
Satterthwaite returns to England and makes straight for the Arlecchino restaurant where he again meets Quin. He tells him that he has failed to get any useful evidence out of the girl but Quin is not so sure and points out the train smoke that she saw. Trains only use the line at ten minutes to the hour and twenty-eight minutes past, therefore the shot could not have happened at 6.20 p.m. Satterthwaite remembers hearing that Sir George is a fussy, habitual man who rewound the house's clocks himself every Friday and realises that he put them back by ten minutes to give himself an alibi, having intercepted his wife's note that morning and realised she and her lover were going to their tryst. He put the telephone out of order to avoid the police logging an emergency call against a specific time. Having heard the maid's hysterical story of the sign in the sky, he realised that she alone had evidence that could smash his alibi and got his secretary to get her out of the country.
Quin suggests Satterthwaite take this evidence to Sylvia Dale who has remained loyal to Wylde all of this time, despite his affair with Lady Barnaby. She goes to Sir George and, telling him a lie that the police now know of Louise Bullard's story, she extracts a confession from him.