Francis Ulrich Tukur is a sharp, sardonic cat who moves into a gloomy, crumbling house with his owner, Gustav Löbel, a romance writer and archaeologist. From the moment he steps onto the top floor, a sharp chemical tang insinuates itself into the air, hinting at secrets lurking above. As Francis begins a quiet exploration, he uncovers the grisly body of a cat killed by a neck wound, a finding that unsettles the already-shadowed atmosphere of the mansion.
There he meets and quickly befriends Bluebeard Mario Adorf, a foul-mouthed, one-eyed, mutilated Maine Coon who has seen too much and says little that isn’t salty. The discovery of another corpse soon confirms that something darker stirs on the top floor: a clandestine gathering of felines led by Joker Ulrich Wildgruber. This sect esteems a figure named Claudandus/Pascal Klaus Maria Brandauer, and they enact ritual suicides by plunging into an electric current, a ritual that stitches fear and awe into the city’s rooftop folklore.
Francis is driven to flee across the city’s roofs, finally slipping out through a skylight and meeting a blind tabby named Felicitas Mona Seefried. She offers critical whispers about the Claudandus sect and its reach. The following day, Bluebeard escorts Francis to Pascal—an elderly, tech-savvy cat who keeps a meticulous roster of feline residents and confirms that Felicitas is the latest victim. That night, Francis is haunted by a nightmare featuring the famed friar and geneticist Gregor Mendel, a chilling echo of the lab-side whispers that haunt the building.
During a routine rodent hunt, Francis stumbles upon a hidden video recording that documents the top floor’s past as an experimental laboratory. The project aimed to develop fibrin glue and tissue adhesive capable of sealing wounds instantly, but most test subjects—stray cats—died in agony. One survivor had been christened “Claudandus” by the technicians, a name that would echo through history. Claudandus eventually murdered the project’s alcoholic and increasingly unhinged lead technician, Dr. Preterius. After the grim events, other subjects escaped, and the lab was shuttered. Claudandus—who would become a martyr in modern feline myth—disappeared into legend, revered by Joker’s faction.
The discovery of hundreds of cat skeletons in a long-forgotten crypt reveals that killings have been occurring for years, hidden in plain sight. Francis begins to suspect Joker, the only cat who ever claimed to have met Claudandus, until Joker himself is found dead. Confronting Pascal, Francis learns the shocking truth: Pascal is Claudandus. After murdering Preterius, Claudandus had been sheltered by Preterius’s former assistant, Ziebold, and learned Mendel’s laws of heredity. Over years he engineered a new breed of cat—imbued with ancient traits—in a bid to resurrect the primal ancestor of all domestic cats and wipe out humanity. The murdered cats had refused to participate in his brutal eugenics program.
Terminally ill with stomach cancer, Claudandus declares Francis his chosen successor. Francis, determined to end the madness, deletes Claudandus’s computer records, a move that triggers a devastating house fire. As flames swallow the top floors, Francis confronts the tyrant in a final struggle and disembowels Claudandus, who mourns the loss of his innocence even as his legacy burns away. In the suffocating heat, Francis manages to rescue an injured Bluebeard from the blaze, proving that survival and mercy endure even in the darkest corners of a haunted house.
