The story opens at the exclusive Grand Babylon Hotel in London, where the self-willed American millionaire Theodore Racksole, infuriated by a head waiter's arrogance, impulsively purchases the entire establishment on the spot from its retiring owner, Felix Babylon. What begins as a whim quickly escalates into a dangerous labyrinth of crime when young Reginald Dimmock, a companion to Prince Aribert of Posen, is mysteriously poisoned after dropping cryptic hints about a secret plot. Racksole and his sharp-witted daughter Nella dive headlong into the shadows of the hotel, discovering hidden passages, locked rooms, and a clandestine network operated by the elusive head waiter Jules and the renowned chef Rocco. As Prince Eugen of Posen mysteriously vanishes abroad on the eve of securing a massive financial loan from the city broker Sampson Levi, the plot thickens around dynastic ambitions and dark schemes.
Nella pursues the conspirators across the channel to Ostend, uncovering a web of secret imprisonment and daring escapes. Meanwhile, back at the hotel, Racksole navigates treacherous waters, uncovering Rocco's macabre activities in the royal apartments and matching wits with Jules in the legendary wine cellars. The narrative weaves a tapestry of high finance, royalty, and criminal ingenuity, culminating in a tense struggle to protect the heirs of Posen from ruin and assassination while preserving the unmatched dignity of the Grand Babylon.