Late 19th century, London.

Rui, a boy living as a vagrant in the back alleys, encounters Sherlock Holmes, a detective, after becoming entangled in a murder case. Despite his resentment of Holmes's indifference to the lower classes, Rui decides to work under him as a "hound" to save the street children caught in the swirling crimes and conspiracies of London—the "demonic city."

Source: MANGA MILLION