The world is in crisis as the Solar System is slowly drifting into an interstellar molecular cloud ("the Encroachment"), which will eventually dim and then destroy the Sun, ending life on Earth.

Hortis Gadfium III, the Chief Scientist of the ruling class, is engaged in a conspiracy with like-minded nobles who believe that the elite are not acting in the best interests of the population, and who question the real motive of the ongoing war with the rival clan of Engineers.

She learns of a message apparently sent from the fast tower, the highest and previously inaccessible part of the castle, which stresses the danger of the Encroachment and tells of an attempt by the crypt to activate a long-forgotten subsystem which may prevent disaster.

Whilst searching for a lost friend, he attracts the attention of the Security forces and takes refuge with various chimeric animals whose implants have taken on personalities from within the crypt.

He is eventually tasked with ascending the central shaft of the highest tower in a vacuum balloon in order to reach its control room.

Asura is able to activate the "Fearsome Engine" of the title, which begins the slow process of relocating the solar system out of reach of the cloud.

Literary critic and historian Ian Duncan has argued that Banks's ‘fearsome engine’, like his The Bridge, “is another allegory of the state, except that this apparatus is not just sublime in its dissociation from human accountability – it is omniscient, providential, and even organic.”[2] Kirkus Reviews described it as an "extraordinary, often brilliantly inventive odyssey," concluding, "Dazzling stuff: a shame it doesn't add up.