The action takes place in London shortly before Christmas. The story's protagonist is a young man named Johnson who works for an eminent lawyer named Arthur Wilbraham. Arthur Wilbrham has been defending a man named John Turk, who was accused of murdering a woman and cutting her body up into small pieces. Johnson is obliged to be in court for every day of the trial, which he finds highly unpleasant. When the trial is over, Johnson is glad that he will not have to see John Turk's face again and is looking forward to going away on a Christmas vacation to the Alps. He asks Arthur Wilbraham to lend him a kit-bag to take with him on vacation. After the requested kit-bag arrives, Johnson passes a fright filled night.
The young man Johnson is the private secretary to the respected lawyer Arthur Wilbraham. In December, Arthur Wilbraham has been defending a man named John Turk who was accused of killing a woman and dismembering her body. At the end of the ten day murder trial, John Turk is found not guilty on the grounds of insanity. Johnson was required to attend the Old Bailey for every day of the trial. He is happy that the trial is over and that he will never again see the face of John Turk, with its white skin, large fringe of black hair and evil-looking eyes. Johnson is now looking forward to going on a Christmas vacation in the Alps. He asks Arthur Wilbraham to lend him a kit-bag to take on his vacation. Wilbraham says that he will get his servant Henry to send the kit-bag over to Johnson's home that evening.
Johnson rents the top floor of a large old house in Bloomsbury. The floor below Johnson's rented rooms is unoccupied. Below that, there are other lodgers whom Johnson does not know. When Johnson returns home that evening, his landlady Mrs. Monks shows him that the kit-bag which he asked for has already arrived. The kit-bag is a large canvas one with a brass padlock. Johnson goes up to his rented rooms and begins packing.
At about ten o'clock that evening, Johnson remembers some items that he forgot to pack. As he goes to fetch them, he hears someone coming up the stairs. He thinks at first that it is Mrs. Monks coming to bring him some mail. The footsteps suddenly stop. It then occurs to Johnson that the footsteps were rather too heavy to have been those of Mrs. Monks.
Johnson continues packing. It is only then that he notices how old and dirty the kit-bag is and sees that it has not been well looked after by its owner. Wilbrahams appears to have been inconsiderate in choosing that particular kit-bag to lend to Johnson. The top of the kit-bag flops down. The bag then takes on the appearance of a human face. Folds in the canvas look like a nose and forehead. The brass ring for the padlock looks like an eye. A shadow or a stain looks like a fringe. It looks just like the face of John Turk. Johnson laughs and says to himself, "That horrid case has got on my mind."
Hearing the sound of someone walking about on the unoccupied floor below him, Johnson goes down to investigate. He turns on the electric light and looks in all the empty rooms. He cannot see anybody there. Johnson goes back to his bedroom and continues packing. He has a feeling that he is doing something to which somebody else strongly objects, not without reason. Johnson does not know why he has that feeling.
Johnson remembers another item that he has forgotten to pack. He goes to fetch it from the living room. Through his open front door, Johnson briefly sees a stooped figure near the top of the stairs that lead to his rooms. When he goes to investigate, the figure is gone. As he is going back to his rooms, Johnson hears footsteps come up quickly behind him and go into the bedroom of his apartment. Johnson thoroughly searches his bedroom but cannot find anyone hiding there. When he steps backwards, Johnson almost trips over the kit-bag. The bag is not in the same place where Johnson had left it.
Having gone into the living room to sit by the fire, Johnson cannot stop thinking about John Turk's murder trial and keeps imagining John Turk's face. He decides to finish packing and then go to bed. When he goes into his bedroom, Johnson sees that the kit-bag has moved again and is now closer to the door. Johnson sees a human head duck down behind the kit-bag to hide. He also distinctly hears a human sigh. When Johnson looks behind the kit-bag, however, there is nobody there.
The kit-bag has come open. For the first time, Johnson sees that there is a large faded bloodstain inside the bag. He falls back in horror. As he does so, he bumps into his bedroom door, causing it to slam shut, and his left arm accidentally turns off the electric light. As he struggles to find the light switch. Johnson can hear the kit-bag moving by itself in the darkness. He also hears the sigh again. When he turns on the light, Johnson sees John Turk standing in front of him. The sighs now form the words, "It's my bag. And I want it." Johnson remembers that he saw the kit-bag before during the murder trial. After John Turk killed his victim and cut up her body, he put her body parts into the kit-bag, filled the bag with quicklime and buried it.
With difficulty, Johnson manages to open his bedroom door and collapses unconscious on the floor outside it. He cannot face returning to his bedroom that night and eventually goes to sleep in an armchair in his living room.
Early the following morning, another one of Arthur Wilbraham's servants comes to see Johnson. The servant brings a clean-looking kit-bag and says that Henry foolishly sent the wrong bag by mistake. Johnson senses there is something else that the servant is somewhat reluctant to say. The servant says that John Turk killed himself at about ten o'clock the previous evening. In his suicide note, John Turk asked to be buried in the same kit-bag in which he had tried to hide his victim's body.
- A kit-bag (also written as Kit bag or kitbag and also called a duffel bag, duffle bag, gym bag or seabag) is a large cylindrical fabric bag with a drawstring closure at the top.