The Vanished Murderer (2015)

A women's prison in the Northern China in 1932, Prisoner FU Yuan (JIANG Yiyan) digs a tunnel in her prison cell with a silver spoon and escapes. Inspector SONG Donglu (LAU Ching-wan) is summoned to investigate FU's escape. By tracking down the letters sent to him by FU, it sets SONG off to Hong City to unravel FU's disappearance.