After some animal activist assholes release infected monkeys from a testing lab, the fearsome Rage Virus decimates England. The virus is essentially a fast acting zombie plague; within moments of exposure, the victim is turned into a mindless, bloodthirsty automaton that kills everything in sight. A scrappy group of survivors try to make their way out of London and into the countryside in search of a mysterious rescue beacon.
28 Days Later is one of my favorite horror movies of this past decade, and what I like about it (and what I suspect many genre fans don't like) is that it is uniquely sentimental and humanist for a zombie apocalypse film. Most zombie films are about defeat, about the loss of hope, about how mankind has to sacrifice its own humanity in order to survive. Even if the ending leaves room for some hope, like Dawn of the Dead, it is only a faint glimmer, somewhat ambiguous. It's such a prevalent attitude in the subgenre that even this one's sequel, 28 Weeks Later, takes the defeatist route.
Not this one. 28 Days Later is about how we need other people to survive. The heroes in this film live because they care about each other, because they form a family unit, because they protect each other. The film is a rare exception amongst horror movies in its willingness to take time out to share sweet, intimate, touching moments with its characters.
The end result is that, unlike so many other horror movies (including a lot of good ones), I actually care about the characters in 28 Days Later. It's not just scary when one of them dies, it's upsetting emotionally.
My boy Patrick has a theory that a good horror movie should leave you feeling like shit, scared and depressed, otherwise it wasn't doing its job. I think it's a fair point for a certain type of horror movie, but 28 Days Later earns its happy, cathartic ending. I wouldn't want it to end any other way.