In the sweltering summer of 1900, young wharf labourer Patrick O'Reilly is down on his luck in the slums of Sydney and homesick for Tralee. When a deadly outbreak of plague descends on the city, O'Reilly’s daydreaming mind is miles away - in the golden hair and kindly, confident air of a girl called Rosie Hughes. Just as he’s wondering why any girl would want a no-hoper like him, opportunity knocks with the offer of a job as a rat catcher working for the city’s Plague Department, containing the spread of disease. But the job will bring him a lot more than a pay rise and a swift education on traps and poisons.

In the Public Lending Library, on the top floor of the Queen Victoria Building, above the bustling centre of Sydney, he comes face-to-face with a legendary rat called Old Scratch who will change the way he understands himself and the world forever.