The un-named English narrator, a collector of antiques, is visited, on the recommendation of a friend in Paris, by Mr X – a fellow collector, an anarchist, and a bon viveur. Mr X explains the complicity between anarchists and certain elements of the classes they wish to overthrow. He describes the Hermione Street anarchist headquarters in London set up in the house of a young middle-class woman supporter. The group take over the ground floor restaurant, establish a business exporting tinned soup on the top floor, and set up a printing press in the basement. But despite good organisation, all the actions planned there are foiled by the police. Mr X arrives from Paris to investigate the possibility of there being an informer in the group. He interviews the Lady Amateur patroness of the group, and meets her admirer Severin.
A plan emerges to blow up the building next door which houses a government department. Mr X is sure that the police will raid the Hermione Street headquarters, so he organises a fake raid by sham police, hoping to flush out the informer. When the bogus raid takes place the group are ‘arrested’ in the cellar, and then are joined by the Lady Amateur and her lover. Severin gives himself away to the fake inspector by trying to protect her, and when his mistake is exposed he takes poison and dies. Mr X then takes the Lady Amateur home.