Hung plays Ding, who we hear through both expository dialogue and seemingly-third-person narration was an elite agent in the Hong Kong equivalent of the Secret Service. He recently witnessed a gang murder and might’ve put a major gang figure away, but in the lineup he couldn’t remember him because “We think he has dementia.” (I feel like there might’ve been a more dramatic way to reveal that information than to just have a cop say it in the opening scene.)
There’s a little Clint Eastwood in the movie’s quiet, gentle portrait of Ding’s lonely life fending off advances from his landlord (Qinqin Li) and mourning his relationship with his daughter, who won’t speak to him because he lost her daughter (to a child murderer?) when he was supposed to be watching her. It’s never fully explained, but seems doubly tragic because we can assume his condition played a part in what happened, but his daughter seems to blame it on him just being a piece of shit.