John Turnbull Angus casually proposed marriage, and Laura Hope casually accepted. However, there was something on her mind that she felt her fiancé should know. She had once served as a barmaid in her father’s inn. The bar had two ugly patrons. One was a dwarf named Isidore Smythe. The other was a tall man with a terrible squint. His name was James Welkin.
Both gentlemen proposed to Laura at roughly the same time. She did not want to marry either of them because of their undesirable physical qualities, but she did not want to hurt their feelings. Therefore, instead of telling them that she did not want to marry anyone who was ugly, she focused on the fact that they were living on inherited wealth. She did not want to marry a man who had not carved his way in the world.
This inspired both gentlemen to leave the village and seek their fortune. As time passed, she received two letters from Isidore Smythe. The first letter revealed that Smythe had made a mark in show business. By the time that Laura received his second letter, he had become a millionaire. He had invented popular robots that could perform household tasks. Smythe boasted that they were butlers that never drank and housemaids that never flirted.
Laura had received no messages from Welkin since the time he left. However, he seemed to be haunting her. Shortly before receiving Smythe’s first letter, she heard Welkin laugh, even though no one was in sight. Later, when she received Smythe’s second letter, she heard Welkin say: “He shan’t have you, though.” No one was in the room.
As they were discussing these things, Smythe entered the scene. Angus confronted him. Instead of arguing or fighting with one another, Smythe directed the attention of Angus to a yard and a half of stamp paper pasted on the window. It was a message addressed to Laura, saying: “If you marry him, he will die.” No one had seen the man who pasted the message on the window, but Smythe recognized the handwriting of Welkin.
On five different occasions, Smythe had found death threats in his flat. On each occasion, the porter insisted that no one suspicious had entered the building. The messages were always in Welkin’s handwriting.
Angus suggested that they consult a clever detective named Flambeau. By chance, Flambeau lived in Lucknow Mansions, which was close to Himylaya Mansions, where Smythe lived.
Angus first brought Smythe to his flat in Lucknow Mansions. He wanted to show Angus the death threats that he had received. To their surprise, the flat contained a new message that read: “If you have been to see her today, I shall kill you.” It was written in red ink that was not completely dry.
Angus went to fetch Flambeau. To protect Smythe till he returned, he asked four men to be on the lookout for any stranger that attempted to enter the building: a worker, the commissionaire whose duty it was to guard the entrance, a policeman, and a chestnut vendor.
Then Angus went to the flat of Flambeau and explained the situation. Father Brown, a short priest, happened to be present. The three men decided to hurry to the flat of Smythe.
Before they entered the building, the chestnut vendor, the policeman, and the commissionaire assured them that no one had entered or left the building. However, it had recently begun to snow, and Father Brown called the attention of his companions to footprints in the snow. Someone had entered the building. Angus thought that he must be an invisible man.
Flambeau and Angus rushed to the flat of Smythe. The presence of blood convinced Flambeau that a murder had been committed, but no one was in the flat, either dead or alive. The invisible murderer had evidently made his victim invisible. Angus even considered the possibility that one of Smythe’s robot servants had eaten him.
Flambeau and Angus left the flat and questioned the people whom Angus had posted as guards. When they looked for the policeman, Father Brown informed them that he had sent the policeman on an errand.
When the policeman returned, he informed the group that he had found the body of Smythe in a canal, where Father Brown had instructed him to look. He had been stabbed.
The policeman had not seen him leave the building.
At Father Brown’s suggestion, they walked a ways down the road. He explained that people do not always give literal answers to questions. If a lady is asked whether anyone is staying with her in her house, she will give a negative reply, even though she has several servants living in the house. She answers this way because the questioner wanted to know if she had any special guests.
In the same way, Laura Hope said that no one was present when she heard laughter and when she heard the voice of Welkin. These statements were literally untrue because someone had to be present to bring her the letter. She had not taken the postman into consideration when she made the statement. In the same way, the four men guarding the building did not take the postman into consideration when they said that no one had entered it. He was mentally invisible because his presence was an expected occurrence. For the same reason, they did not take notice when the postman left with the body of Smythe in his mail sack. When a postman passed with a mail sack, it simply would not attract their attention.
Father Brown had led Flambeau and Angus to a place where they would meet the postman. When they confronted the postman, it was obviously Welkin. He had an almost fiendish squint.
Out of pastoral concern, Father Brown went for a walk with the murderer and conversed with him at length. Chesterton does not record the conversation or its results.
Since I no longer have access to the book, my summary has been based on a version presented online by the Project Gutenberg.
Gutenberg Project: The Innocence of Father Brown