Is it possible to be both impressed and disappointed by something? That was my great takeaway from Danny Boyle's long awaited return to his genre-defining zombie film, 28 Days Later.

For over two decades, we've waited and dreamed about where the story could go next. Turns out, it's right back to where it all started. Instead of spreading into continental Europe, 28 Years Later looks inward, returning to the working class neighborhoods and small lives of the average Briton caught up in the tide of bloodshed.

At its best, Boyle and writer Alex Garland paint a sweeping image of what the world looks like when an entire island nation is faced with insurmountable trauma, and how a collective regresses into the safety of the past.

These sections are the highlight of the story, yet they feel like elements Boyle and Garland are in the greatest haste to leave behind. We follow a boy, Spike (Alfie Williams), and his father, Jamie (Aaron Taylor-Johnson), as they head out on a rite of passage to the mainland. Their home is an island within an island, separated by a flooded causeway. There, they've built a new society from scraps of memory, nostalgia, and patriarchy. When boys hit puberty, they go out with their fathers to kill the infected, and thus become men.

But while their world has come to a standstill, the mainland has continued to evolve. The infected aren't just surviving, but becoming something else. New subcultures have formed, while a terrifying breed of Alpha infected seemingly lead the others.

As a setup, it's a fascinating continuation of the apocalyptic vision. Where 28 Days Later ended on a promise of rebuilding, that one day the structures we've put in place will prevail, 28 Years Later presents a far bleaker and wearied reality. Multiple economic collapses and a global pandemic later, it's hard to believe in the power of society when it has proved itself so brittle.

Yet within minutes, Boyle and Garland forget about their initial concept and focus on a much leaner, more brutal story of survival, which in turns morphs into another coming-of-age tale, that once more gives way to the promise of what the intended trilogy will become. At two hours in length, 28 Years Later packs in no less than four different narratives, one which bookends the story in a greater – and entirely unexplored – mythology.

It's a busy film with loads of ideas and little time to explore them with the nuance they deserve. Where the first outing was strictly limited by budget and time, it was also a stronger narrative in its depiction of working class unity and colonialism than anything 28 Years Later can muster. Even as it draws immediate comparisons between films by introducing a half-baked plot point involving a stranded Swedish marine into the mix.

Even Ralph Fiennes, who turns up in a scene-stealing role, feels like he's here to open the door for future appearances. His presence raises more questions than it answers, and he brings much-needed gravitas to some of the more absurd notions Garland's script entertains. But he's in and out of the picture within ten minutes. In a TV series, he'd be promoted to a series regular next season.

In the final act, Garland and Boyle lose faith in their new world. Like the islanders, they fall back into nostalgia, and repeat music cues from the first film, along with clear visual callbacks that now feel like gimmicks. 28 Days Later looks unique because it was shot on consumer grade cameras to save money and time. 28 Years Later has the backing of a major studio behind it and budget to spare. Now, the use of iPhone's draws attention to itself in the wrong way.

Despite this, 28 Years Later is a fascinating film. When it works, it redeems every expectation audiences may have built during the long wait between movies. It is an undeniably bleak story, one that weighs heavier on the soul the more you think about its implications. There's an angry, defeated air to every minute of it. As a horror film, it captures the sense of biblical dread better than almost any other in this genre.

But it's only a third of the big picture, and it doesn't stand on its own. By the end, it feels unfinished, and, as such, unsatisfying. As a writer, Garland has a tendency to craft magnificent openings that never stick the landing. Sunshine, the sci-fi epic he and Boyle made in the mid 2000s, comes to mind.

28 Years Later isn't there yet, but you should approach it with the understanding that it's only an initial pitch. A tantalizing, often brilliant, and mostly uncompromising promise of what's to come. The sequel – The Bone Temple – arrives 28 weeks later, in January. The finale is still being written.

In a way, it's a strange feeling to be this optimistic about something so bleak. Whatever the future holds, it won't be boring.