St John Greene's Mum's List, a bestseller from 2012, is a tale of awful luck and impressive resilience. The author – known to all as Singe – watched helplessly as two of his immediate relatives contracted cancer. His son Reef, diagnosed at just 18 months, defied wretched odds and survived into boisterous youth.

His wife Kate, despite a more hopeful prognosis, died in 2010 from breast cancer. In the months before her passing, Kate put together 100 snippets of advice that she hoped would keep her spirit alive – including a suggestion that Sing move on and see other women.

Greene’s book has been an inspiration to many. Unfortunately, the film is an incoherent mess that may prove baffling to those unfamiliar with the source. Two fine second-generation actors deliver committed performances.

Rafe Spall (son of Timothy) and Emilia Fox (daughter of Edward) are, respectively, crumpled and resilient as Singe and Kate. The juvenile actors throw themselves at their roles with commendable gusto. None of that good work can distract from the shallow characterisation and criminal lack of structure.

Kate is defined solely in terms of her illness. Sing is defined solely in terms of his response to her illness. What does he do for a living? What are his interests outside the family? At one stage, out of nowhere, his parents turn up and then vanish again almost as quickly.

More seriously still, the film can’t decide what it is doing with the titular list. We begin with the protagonist, as instructed by Kate, kissing his sons twice as they leave school. Later, random attempts to satisfy the requests are interspersed with flashbacks to Kate’s diagnosis and decline.

Or should we call the later scenes “flashforwards”? The temporal shape is so disordered that it becomes impossible to sustain momentum. Put simply, Niall Johnson, the director and screenwriter, has failed to make a story of Greene’s book. A disappointment.