"28 Days Later" the latest film by director Danny Boyle ("Trainspotting", "Shallow Grave", "The Beach"), is an unashamed and unabashed tribute to George A. Romero's zombie trilogy, "Night of the Living Dead", "Dawn of the Dead", and "Day of the Dead". Boyle, however, takes great pains to point out that his film is not about zombies.

The film begins in a medical research lab. A trio of animal rights activists break in to liberate the test subjects, and unwittingly unleash a virus into the outside world. Time passes (the 28 days of the title, specifically), and a bicycle courier named Jim (Cillian Murphy) emerges from a coma to discover that not only the hospital, but the entire city of London, has been abandoned.

Jim finds that the walls of the city are covered with notes and pictures, posted by people who want to know where their loved ones are, or perhaps by people who just wanted to leave some proof that they once existed.

In short order, Jim discovers that the city is not as empty as it seems. The virus drives an infected person into a homicidal fury, and once an infected person's blood or saliva enters your system, you're doomed to their fate.

Luckily for Jim, he finds other survivors, including Selena (Naomie Harris), who could give Ellen Ripley from "Alien" competition as woman you'd most want on your side when the apocalypse begins.

What makes "28 Days Later" worthwhile is the time spent on establishing who the people we're watching are. Unlike the recent backwoods horror film "Wrong Turn", the characters are not merely horror cliches falling in predictable order. The humanization of the characters accelerates rapidly once Jim and Selena encounter Frank (Brendan Gleeson) and his daughter (Megan Burns), who have picked up a radio broadcast from Manchester, telling them safe haven and an "answer to infection" awaits.

To tell more would be spoiling a good story; if you're easily offended by violence or gore, you'd be well advised to skip "28 Days Later". If you appreciate a well made horror film (something which is unfortunately rare), this is required viewing.