When in 1936 a brutally tortured body of a local postman is found outside of Shanghai's St. Theresa's church, the local constable brushes off the case. Zhongliang, however, comes to find out that his father's death was no mistake. Giving up his studies, Zhongliang stumbles into a secret world where he is known as 'Catfish'. His new postman duty takes him to the streets of Shanghai where he delivers coded letters to cryptic addresses, conspires against the invading Japanese forces, falls in love, and eventually gets completely absorbed by his own sense of righteousness.