Our hero is a package delivery guy by the name of Jim (Cillian Murphy, a.k.a. the Irish Jim Caviezel), who has been in a coma just long enough to miss the whole breakdown-of-civilization thing. Given his profession, the film's title actually takes on an extra significance for Britons: Much like "6-8 weeks" here, "28 days" is the standard amount of time UK mail-order companies tell consumers to allow for their products to arrive.
Jim awakens in a hospital, and finds it bereft of human life. He has been locked in his room, which is presumably the only reason he remained, but fortunately the key is right under the door. After chugging a Pepsi (the joy of life he needs at any cost), he emerges onto the streets of London and finds no one there either. Message boards are overladen with handmade "Have you seen this person?" signs, and the nearest church is filled with dead bodies. Or are they? One or two are in fact extremely mobile and clearly malevolent. Technically, they're not "old school" living dead -- they're living humans infected with a virus and can be killed by normal means. Unlike most of their cinematic predecessors, however, they move like wild animals on speed while still boasting the standard living-dead contagion factor in their saliva and blood.
Jim encounters a few other survivors, notably hard-edged survivalist Selena (Naomie Harris) and a middle-aged taxi driver (Brendan Gleeson) with a teenage daughter (Megan Burns). When they come upon a radio message telling them that the answer to the infection has been found, they head off in the still-operational taxi to find the source, but it's hardly the salvation they were hoping for. Without spoiling too much, let's just say that times of trouble can bring out the worst in mankind, which can be as frightening as any monster.
Director Danny Boyle (Trainspotting) opted for digital video for matters of practicality, mainly because it's difficult to evacuate whole sections of London for the length of time it would take to shoot on film. Thus, the film mostly has a low-res look, save the final scene, shot in broad daylight in a manner that could have been done on film. Abetting him in the cinematography department is Anthony Dod Mantle (The Celebration, julien donkey-boy, Mifune), whose cinematography has traditionally been the weakest part of the projects he's been involved with. But unbound by ersatz "purity" vows, he probably had the freedom to use artificial light this time. Some scenes are too underlit, but in general, the film looks decent blown up on a big screen and certainly better than its low-grade trailer indicated.
The cheap camera also does for Boyle what it did for Joel Schumacher in Tigerland -- both men had become bogged down in silly visual excess in their previous films and had to get back to the basics of story and character. For Boyle, this means a movie more like Shallow Grave than any he's done since, and not just because Christopher Eccleston returns to play the most morally ambiguous character. That dark wit is back: Gleeson gets to be positively Homer Simpsonesque in a supermarket scene wherein he discovers the sole batch of apples that has not yet spoiled and goes "Mmmm... irradiated!"
One can nitpick about certain details. How, for instance, does a package delivery man suddenly become a deadly assassin capable of taking on trained soldiers? Why, when you know zombies favor darkness, would you opt to go down an unlit tunnel rather than take the long way around? And why, once you've done that, would you stop to change a flat tire in the dark tunnel rather than maybe driving on the rim for a little while longer? On the latter two questions, Jim offers a rebuttal of sorts -- "This is a shit idea. You know why? Because it's really obviously a shit idea!" -- but on the first point, it seems to be that Boyle is making some sort of point about human rage, since the virus is also called "Rage." They are us, and we are them, you know. Whatever. The deep thematic concerns are never fully developed, but the characters are, and the story compels.
Also, the movie's pretty scary. That's what counts.