Kowloon Sousa Kaikiroku

The manga centers on Torii, a Japanese citizen who arrives in Hong Kong's Kowloon Walled City for a certain reason. Getting lost within the enclave until after dark, he meets a man named Sharaku who asks him if he's Japanese. Sharaku then proceeds to kidnap Torii, and he becomes involved in a mysterious incident.

(Source: Anime News Network)