When he was 24, film-maker Kit Vincent was diagnosed with a brain tumour; doctors said that he could expect to live four to eight years. This emotional, raw and quietly powerful documentary started out as a study of how his dad Lawrence came to terms with his son getting ill. The title is a giveaway that the finished article is not that film.

At times, it feels like family therapy. Vincent hangs out with his parents, who divorced when he was a teenager. Time is running out, and the camera is switched on – two facts that force everyone into the kind of deep, soul searching conversations that most of us spend a lifetime avoiding having with family. Lawrence (tense and distant-looking in old family photos, mellowed with age) was in the hospital room when Vincent got his diagnosis and promptly had a heart attack. The guilt is still with him: “Just when you needed me the most I flaked out on you.”

Lawrence has found ways of dealing with what’s happening – he’s converting to Judaism, started painting, and at one point he turns the spare room into a farm to grow medicinal cannabis. Vincent’s girlfriend Isobel is a reluctant subject. When he brings up sperm-freezing she angrily accuses him of only ever wanting to talk about important stuff on camera for the sake of the film. Another time he ambushes her: “Will you get a new boyfriend when I die?” It’s disarmingly candid of Vincent to include these moments where he doesn’t come over well – and they give a glimpse too of how alone it must feel to be where he is.

On the voiceover Vincent says he hopes that the film will be something for his family to remember him by. I don’t think anyone says it in so many words, but of course Red Herring will be his professional legacy too. It is a very good film, the work of a really talented film-maker.

• Red Herring is in UK cinemas and on digital platforms from 3 May