This is not how franchise filmmaking is supposed to work. The traditional studio-driven model demands regular sequels, returning characters, and perhaps a careful insertion of some new ideas each time – albeit still underpinned by an overall familiarity dictated by a successful first film.

Danny Boyle’s 28 Years Later ignores some of these rules. Others it seems to actively subvert. This third part of the 28 saga does not come hot on the heels of the second: it has been 18 years since Juan Carlos Fresnadillo’s military thriller 28 Weeks Later. Neither is the film a nostalgia-infused legacy sequel. No characters return from earlier films. In terms of tone and genre, it does not really match either previous instalment. 28 Days Later (2002) combined the zombie movie with a very British ‘cosy catastrophe’ (its relationship to author John Wyndham sits somewhere between ‘heavy debt’ and ‘blatant plagiarism’). As noted, 28 Weeks Later (2007) adopted more elements of a military thriller: larger in scale, and more muscular in terms of firepower and violence. For the belated third time around, the film owes less to those earlier influences and a lot more to rural drama and folk horror. Its relationship to the zombie genre – something Boyle has always denied, or at least heavily underplayed – is at its most tenuous. I suspect some fans of the earlier films will be disappointed by this one. I think others will be delighted. This is a much more interesting film than I expected, to its enormous credit.

Twenty-eight years after the rage virus decimated Great Britain, 12-year-old Spike (Alfie Williams) lives with his parents on a well-defend island community off the Scottish coast. His father Jamie (Aaron Taylor-Johnson) scavenges from the mainland. His mother Isla (Jodie Comer) is debilitated by a developing neurological illness. When Spike learns of a rumoured doctor living alone on the mainland, he steals away from home to get Isla the help she needs.

The mainland remains infested with those infected with the rage virus that brought about Britain’s collapse in the first place. They are not as viewers remember them: they have evolved. Rather than die of starvation, as assumed by the end of 28 Days Later, they have continued to survive. They have started to breed. They show signs of their own rudimentary language and culture. Some have transformed into slow crawling beasts, sniffing the undergrowth for worms and insects. Others have becoming towering, muscular ‘alphas’, stronger and faster than regular humans with a taste for flesh. They do not really feel like zombies any more. Given the rural setting, they take on a more fantastical purpose: they are the monsters than fill the haunted forest.

There is a different purpose to this story that its predecessors. They, like many horror films, were about escape. There is no escape from infected Britain. International patrols guard the coast, keeping everybody quarantined. On the mainland there is nowhere to hide. The best hopes of Spike’s Lindisfarne home is to survive and remained protected. Instead, at its core, 28 Years Later is a very small and personal drama about love, family, and accepting loss.

The cast are exceptional. Alfie Williams is deeply emotive and expressive as Spike: reluctant, fearful, and yet resolute. Jodie Comer and Aaron Taylor-Johnson play their roles are real and flawed people. Ralph Fiennes is particularly good, although knowing of his eventual presence in the film does rather kill the suspense over whether or not the fabled Dr Kelson is going to appear.

The film also boasts a rather bold line in social commentary, with its depictions of an isolated Britain no longer connected to Europe, of jealously guarded borders, religious faith, and national identity. Boyle uses the more intense action sequences to cut in battle scenes from Laurence Olivier’s Henry V (1944) – possibly Britain’s most patriotic film – to deeply ironic effect. The climactic emergence of a roving cult dedicated to an unlikely icon is, I’m sure, divisive, but I will spare my own commentary on it for when I see more of them in next year’s 28 Years Later: The Bone Temple.

Aye, there’s the rub. For all of its exceptional qualities, 28 Years Later ends on something of a cliffhanger and never quite resolves its own story. There are obvious hooks left in for a sequel – one shot concurrently with this film – and that does leave a lingering distaste as the credits roll. It may be that The Bone Temple will be sensational, and together with a planned third film will create a lasting and deeply satisfying movie trilogy. For now, however, there is the unavoidable sense of having only enjoyed half a story.