This film starts with a horrific crime: without warning, a 15-year-old boy stabs his mother and sister to death with a kitchen knife in their Hong Kong apartment. Based on real events, this intelligent, understated drama asks the question that’s on everyone’s lips: why? It tells the story of the killings through the eyes of the boy’s father, a man of few words named Yuen, played with tremendous subtlety by Sean Lau in a performance that glimpses at the grief and guilt behind his character’s often expressionless face.

The film moves quickly back and forth in time. In the present, Yuen is coming to the end of a night shift at the family’s 24-hour restaurant when he spots police in the apartment block opposite where he lives. Yuen’s son Ming (Dylan So) has killed his mother Yin (Jo Koo) and 12-year-old sister (Lainey Hung). In the days afterwards, Yuen goes through the motions, picking coffins for his wife and daughter, organising the funeral. His mind drifts back to Ming as a boy, running into nursery on unsteady toddler feet, speeding on his first bike.

At the trial, the court is told that Ming has schizophrenia, and heard voices in his head before the killings. Yuen casts about in his memory for telltale signs in Ming’s behaviour, none of it definitive. He is an insular, awkward kid, like a lot of kids. And like a lot of teenage boys, he spends too much time playing video games. Then there is Yuen’s parenting; he loves his children, but is often distracted, can be irritable and impatient. Should he feel responsible? Could he have prevented the tragedy?

Papa is perhaps too nuanced and low-key to try to answer these questions fully. At the heart of it is Lau’s naturalistic performance. There’s one brilliant moment, very soon after the killings, when he looks out from his restaurant, as if he cannot comprehend how the world is still turning; it is conveyed by Lau with barely a flicker, a tightening of the jaw, a slight squint of the eyes.

• Papa is in UK cinemas from 28 February.