“28 weeks later” could easily have been a complete mess.

It’s a sequel of director Danny Boyle’s and writer Alex Garland’s acclaimed zombie flick of 2002 “28 days later,” helmed by another director Juan Carlos Fresnadillo.

It tells the continuation of a story that was fairly self-contained in its first installment. And then again, it is the follow-up of a post apocalyptic zombie movie, a genre that isn’t really renown for its quality storytelling.

Despite all these initial misgivings, “28 weeks later” is a strong movie that stands on its own merits and manages with the difficult task of telling its own story while furthering the mythos of the first movie, and adding some political commentary just for the heck of it.

Six months have passed since the rage virus ravaged England. The infected, who aren’t treated as the living dead, instead they are merely very sick cannibals, whose ailments are highly contagious, have all died of starvation.

The survivors Don (Robert Carlyle) and Alice (Catherine McCormack) are hiding out in a farmstead out in the country together with a few other lucky individuals.

Before long, a band of infected attack the farm, and Don is the only one who makes it out alive. His flight from the zombies is desperate and extremely intense, shocking the viewer immediately with what is at stake.

Back in London, American troops have seized the city and are working hard to repopulate the capital.

Don’s children Andy (Mackintosh Muggleton) and Tammy (Imogen Poots) arrive back to their ravaged city, and for a moment, everything seems like it’s going to work out. But during an ill-fortuned return to their former home, a survivor of the zombie infestation is found. A survivor who bears dire consequences.

The camerawork in “28 weeks later” uses a lot of the now obligatory gritty and shaky shots to create a visceral experience that has been extremely overused in horror movies. In this sequel though, the tired tricks are put to good use and actually manage to give the movie an appropriate and original look. They lend the movie a sense of alienation and claustrophobia, even in the open outdoors locations.

Many of the shots show the desolate vistas of the dead and abandoned city of London are beautiful in an eerie way.

“28 weeks later” uses actors that aren’t widely famous, the biggest stars being McCormack, Carlyle and Rose Byrne. This lets the characters come alive through their anonymity instead of relying on recognition of a well-known cast, something that is even more so beneficial in a horror movie.

It is fun to see Byrne in a heftier role where she isn’t just a pretty face to look at. She pulls off the part of the competent medic officer Scarlet, despite having some tricky lines about the nature of the zombies and the possible cure.

All of the portrayals in “28 weeks later” are solid without stealing the limelight from each other. One of the biggest accomplishments in this zombie movie is that it actually makes you care for its characters by lending just enough of humanity to all of the people involved.

Another strength of “28 weeks later” is it takes itself serious enough to be that kind of movie where you actually do care about the fates of the protagonists.

The score is well put together, using reoccurring and monotonous compositions to create a sense of looming claustrophobia and panic.

The political undertones are an obvious parallel to the war in Iraq, but it stays in the background and doesn’t force feed the viewer of its stance. Instead it lends the movie a touch of ambiguity which is always welcome.

“28 weeks later” is a gory and intense zombie movie that does what it set out to do, and then a little bit more. If the tossup is between “Spiderman 3” and this movie, make the wise choice and pick zombies instead of radioactive spiders.