"The Invisible Man" is a detective story written by G. K. Chesterton which concerns the nature of the crimes committed by one James Welkin, who had been harassing Laura Hope and menacing his romantic rival, Isidore Smythe (whom he would later murder), all without ever being seen.

The central point-of-view character is John Turnbull Angus, who, at the beginning of the story, proposes marriage to Laura himself. Laura responds to his proposal by telling a story of her own. Some time earlier, she had worked at an inn, where she was met with two unwanted proposals, made by Smythe and Welkin. Not wishing to admit the true reason for her rejection (that she found each of them "impossibly ugly"), she instead answered that she did not wish to marry anyone who had not managed to make a livelihood through his own efforts, as a matter of principle, and gave this as the reason for her rejection.

Both men then left, intending to meet her requirements. Smythe proceeded to make his fortune by inventing robots programmed to perform domestic chores. Welkin, on the other hand, disappeared and became the "invisible man" to whom the story's title alludes, harassing Laura while remaining unseen. Laura dreads the reappearance of both these two men, as she still does not wish to marry either of them.

It is at this point that the character of Smythe, who has himself been receiving threatening letters from the same Welkin, appears in the story. Smythe takes Angus back to his mansion before Angus leaves to enlist the help of his friend Flambeau. In the process, he runs into Father Brown, a Catholic priest and amateur detective who has been training Flambeau in the art of detection.

The three rush back to Smythe's house, only to find that Smythe himself has disappeared, without any witnesses to the crime (even though Angus had charged people to watch the premises when he was gone). Smythe's body will later be found in the canal. Father Brown proceeds to reveal that Welkin had in fact been a postman, and this was what allowed him to go about unnoticed, allegedly delivering the mail. Welkin was able to enter and exit the house without anyone the wiser, precisely because there was nothing out of the ordinary about a postman doing so. (The same logic applies to Welkin's harassment of Laura.) Furthermore, after murdering Smythe, he was able to put his victim's small body in his mail sack, to deposit it in the canal.