When the story opens, we are in the home lab of a bacteriologist. The bacteriologist has a visitor, a “pale-faced man” with “lank black hair and deep grey eyes…[a] haggard expression and nervous manner.” The bacteriologist is showing his visitor a slide of dead cholera bacteria, and the man shows such a keen interest that the scientist shows him a vial of the living bacteria. The scientist begins to wax apocalyptic about the effects of even a single drop of the vial finding its way into London’s water supply, and at this tale of devastation his visitor’s eyes gleam with a poorly-veiled desire. At this moment the bacteriologist’s wife calls him into the hall for a quick word, and when he returns his visitor takes his leave.
It is only after the visitor has departed that the bacteriologist realizes the tube of cholera bacteria is missing. After frantically searching and patting his pockets, he comes to the conclusion that his visitor must have taken it—and he flies out the door in pursuit, still wearing his dressing robe and his house slippers, one of which he loses along the way. His visitor spies him coming just as the former is getting into a cab (horse-drawn, in this era), and with a word to the cabbie he gallops down the street. The bacteriologist himself catches a cab and gives chase. Minnie, his wife, having witnessed this whole episode from the window of their home, thinks her husband mad for leaving without a proper hat, coat, or shoes, and catches her own cab to take them to him. We have a Victorian three-cab chase on our hands.
During this chase we learn that the pale-faced man is in fact an anarchist intent on wreaking havoc throughout London with the stolen cholera (he expresses disbelief at the beginning of the story that anarchists would resort to bombs when they had access to this sort of physical devastation). In the midst of a bumpy ride, the man accidentally shatters the vial in his hand, and faced with the failure of his plan he drinks the few drops left, stops the cab, and climbs out. Spying the bacteriologist pull up behind him, he cries, "'Vive l'Anarchie! You are too late, my friend, I have drunk it. The cholera is abroad!’" and retreats to single-handedly infect the city.
The bacteriologist watches contemplatively as his guest jostles and coughs all over pedestrians, finding the scene more curious than concerning. When his wife catches up, he thanks her for bringing his things and tells her that what the man has drunk is not actually cholera—the scientist, noting the man’s interest in the dead strain of the disease, decided to add a bit of intrigue to the tour, telling his guest that what was actually a disease that turned monkeys blue was cholera. A harmless but unsightly disease. The results of the ordeal the scientist can only guess, but in any case the anarchist will be sorely disappointed.
This story is a brilliant exercise in irony; we have a man who is not who he seems, and a disease that is not at all what it seems. And as the story unfolds we have a situation that could morph into a devastating comedy before our eyes.