Brentford Orsini has just handed over the reigns of his regency to a political enemy, but that's democracy for you. That enemy sends Brentford and six of his friends and allies to Paris on a made up diplomatic mission. Orsini can't refuse because his other option is prison. Of course the mission goes wrong because Brentford has been ordered to take the most dangerous and unreliable means of transportation. The Psychomotive, piloted by a former explorer who has been reduced to being nothing but a head kept alive by machinery, ends up transporting the diplomats back to 1895 Paris.
Brentford and the diplomats spend the rest of the book trying to figure out how to travel in space and time to get back to their beloved, improbable city. While they are in Paris, they become involved in a conspiracy. In 1895, the founders of New Venice have only just begun their plans to create a Hyperborean city. Brentford and his friend, Gabriel d'Allier, can't resist trying to learn more about these founders because the origins of New Venice have been lost to them over time. Meanwhile, the diplomats run afoul of virulently nationalist elements of the French police. Valtat ratchets up the tension by killing off several major characters.
The charm of Luminous Chaos lies in the dense and dazzling world building Valtat does. The diplomatic mission arrives inauspiciously in Paris through the Montparnasse Train Wreck. They run into members of the Parisian literary scene in delightful cameos from August Strindberg and Marcel Proust, among others. They visit the catacombs and seances. On top of all this, Valtat layers a generous helping of mad steampunk science and occultists meddling with forces they ken not. Luminous Chaos is a book to be savored.