This is a novella which rewards your imagination, creativity, and openness to experience. There is a plot, after a fashion, though as usual with this author, it's the journey not the destination. We soon learn of two timelines in the alternate reality wherein resides 'New Paris'.
Vichy Marseilles in 1941 sees a gathering of Surrealists fighting fascism through their works. The atmosphere positively fizzes with their dangerous, subversive 'art of the unconscious'. To hand is an American scientist (and occultist), Jack Parsons, who will bottle this energy in a 'battery' of his own devising to further his own plans to fight fascism. Unfortunately, the thoroughly-charged battery is stolen and ends up in New Paris where it detonates: the S-bomb.
In New Paris 1950, the city is blockaded as Surrealist manifestations stalk the arrondissements aided by the Surrealist underground, La Main à plume. Trapped Nazis and Gestapo agents contract with the denizens of Hell to combat them. The Germans have a top-secret plan: Fall Rot. But what is it? Thibaut, a leader of La Main à plume, has to find out.
The afterword purports to document a meeting between an aged Thibaut and Mr Miéville in a London hotel where the story is recounted in frenzied haste. At the end you will find detailed notes on most of the surrealist works of art which 'manifested themselves' in New Paris. Of particular note is the Surrealist publication/manifesto, "Le Surréalisme au service de la révolution", seeming to serve as a blueprint for the S-bomb.
You may spend most of the novella wondering where all this is going. It does, however, resolve; there is an overarching theme. I might wish it had been a little more subtle than the banality of evil. However, Miéville can make even Manicheanism look chic.