Dear Zachary: A Letter to a Son About His Father is a 2008 documentary written, directed, shot and edited by Kurt Kuenne. It concerns the life of Kuenne's lifelong best friend, Dr. Andrew Bagby, and it was made with a single, specific purpose: so that Andrew's newborn son, Zachary, would one day be able to sit down and understand exactly who his father was. It is one of the saddest and most gut wrenching things I have ever watched, and I am giving it a perfect score.

I want to be firm about the no spoilers policy here, because it matters more with this film than almost any other. Do not read about it. Do not look up the case. Do not let a well-meaning friend explain it to you. Everything that makes this documentary land the way it does depends on you arriving with nothing. So I am going to talk around it entirely.

Here is the background you can safely have. Kurt Kuenne and Andrew Bagby grew up together in the suburbs of San Jose, California, and were close friends from childhood onwards. As a boy, Kuenne made amateur films, and Andrew acted in many of them. As those films grew more ambitious, Andrew believed in them enough to put some of the money he had been saving for medical school into funding them. He went on to become a doctor.

When Andrew was murdered, Kuenne did the only thing he could think to do. He got in a car with a camera and travelled across the United States, into Canada and over to the United Kingdom, sitting down with everybody who had ever loved his friend, gathering their memories into a portrait. It was never conceived as a film for the public. It was a letter, addressed to a child who would not otherwise get to know his own father.

The craft is worth talking about, because it is genuinely distinctive. Kuenne edits at a relentless, rapid-fire pace, cutting hard between interviews, archive material, photographs and old home movies. It is not a calm or contemplative documentary. It moves at you constantly, and the effect is that it keeps finding new ways to break you down. Every time you steady yourself, it locates a fresh angle of attack. That style has been criticised in some quarters as manipulative, and I understand the argument, but I do not agree with it. This is a film made by a man in grief about his murdered friend, for the child that friend never got to raise. The rawness is the point.

It also made me furious, and that fury is directed at the systems and the institutions that had every opportunity to act here and repeatedly chose not to. This was not misfortune. It was a chain of failures, each one avoidable, each one compounding the last. I will say no more than that.