Mercy Chan is a triad exorcist with a mysterious past. After washing up on the shores of Hong Kong with no memory during World War II, she found a home in Kowloon Walled City, an infamous, ghost-infested slum full of lost and traumatised civilians. Since the war ended, Mercy has rebuilt her life and found work as a ghost-talker for the local triad, dealing with the angry and bitter spirits who haunt this place.But the past she can't remember won't let her go.

An unusually powerful ghost lurks in Kowloon's waterways, drowning innocents and threatening the district. Unnervingly, it claims to know Mercy - and her forgotten childhood.As Mercy is drawn into a deadly cat-and-mouse game with this malignant spirit, she begins to realise that the monster she fights within these walls may well be one of her own making.