4 ideas
Fang Yan, a jobless Beijing drifter who fills his days with drinking, gambling and all-night card games, learns he is suspected in a years-old killing and cannot remember what he did that night. The novel is a concrete case of a man who has to work out his own guilt from outside, the way a detective would.
When a person's own recollection is missing, the past gets assembled from the accounts of other people. Each witness has their own interests, gaps and lies, so the rebuilt version reflects whoever was willing to talk rather than what happened. A person can come to accept a story about themselves, even one that makes them a murderer, because it has been told to them consistently.
A life without work, purpose or consequence creates an appetite for anything that makes the heart race: gambling, provocation, even danger to oneself. Seen this way, the protagonist's pull toward the murder mystery is less a search for truth than another high-stakes game to fight boredom. The same need that drives the card table drives his investigation.
When someone cannot say with confidence what they did, guilt and innocence stop being facts they know about themselves. Instead they become verdicts that could go either way depending on the available evidence and who is telling the story. The self shrinks to a set of plausible scenarios, and whether the person could have done it matters as much as whether they did.