Vespera Luminaire is a pioneering radio engineer at the BBC, blessed-or cursed-with synesthesia: she sees electromagnetic waves as ribbons of color and light. Alone in her laboratory one midnight, Vespera intercepts a signal that defies all science: the voices of the recently deceased. She learns a terrifying truth: consciousness survives death as electromagnetic energy, and a mysterious figure known only as The Collector is trapping these souls to power a dark machine called the Eternal Engine. Vespera is his next target, marked for her unique ability to see the spectrum of the dead. Forced into a desperate alliance with occult antiquarian Aurelius Grimwald, psychic medium Minerva Blackheart, and the skeptical Detective Inspector Marcus Nightingale, Vespera must race against the Collector to uncover the real purpose of the Engine: not eternal life, but mass psychological control. The trail of suppressed scientific research leads from the shadowy backstreets of Bloomsbury to the highest echelons of Whitehall, where powerful patrons are waiting to weaponize the ultimate secret: the human mind itself. To save London's free will, Vespera must use her impossible sight to fight a monster who has mastered the frequency of the soul, before the whispers silence the world forever.