 The novel centres on the eponymous "Mechanicum", a cult of machine-worshipping technologists based on the real-life planet Mars and which serves as the chief engineering authority in the nascent Imperium.The machinations of Horus and the Chaos-worshipping Traitors affects the Martian cult as much as every other Imperial organisation, leading to a civil war on Mars itself.Kelbor-Hal, Fabricator General of Mars and the technocracy's supreme leader, declares for Horus, and together they carry out a coup d'état to eliminate Techpriest and Magos adherents of the Cult who are loyal to Terra and the Emperor.