Animal rights activists release a chimpanzee being used for medical research at the Cambridge Primate Research Center, ignoring warnings that the chimp has been exposed to Rage, an incurable virus that instills a murderous frenzy into those infected. Twenty-eight days later, bicycle courier Jim (Cillian Murphy), who was injured in a cycling accident prior to the release of the virus, awakens from a coma in an abandoned hospital in London to find that the city has been deserted and that he is being pursued by "infecteds" who want to kill him. Jim manages to find more survivors, including Selena (Naomie Harris), Frank (Brendan Gleeson), and Frank's teenage daughter Hannah (Megan Burns), and the four of them attempt to make their way up to Manchester where they hope to find sanctuary among a military encampment.

The film is based on a screenplay written by English novelist and screenwriter Alex Garland. A sequel, 28 Weeks Later (2007) followed in 2007.

Based on the traditional definition of a zombie (a reanimated corpse), no. However, the idea of what constitutes a zombie has changed over the years through various forms of entertainment, including movies, TV shows, comic books, video games and more, and the definition is hotly debated among zombie fans. Director Danny Boyle and scriptwriter Alex Garland both feel that the movie does depict zombies, but in a unique way not before seen; according to Boyle, "I feel there was respect for the genre, but I hope that we freshened it up in some way" (production notes, archived here). With this in mind, "The Infected" are neither the traditional "zonbi" of Haitian folklore, the living-dead of old Hollywood monster movies, nor the Romero-styled re-animated corpses that feed on uninfected flesh. But they are mindless drones who act in numbers, rather than individually. They do not eat, speak, rationalize, form new ideas or even determine how they will make their next move, instead acting purely on base instincts, and in this sense, they act very much like traditional zombies.

The film is somewhat ambiguous as to its sub-genre, leaving it to the viewer to make the final classificatory decision. The film could easily be included in traditional genres such as survival horror, horror drama, zombie, virus, and post-apocalyptic.

The movie does not say. However, the graphic novel 28 Days Later: The Aftermath, which begins prior to 28 Days Later, as well as bridging the timespan between the events of 28 Days Later, and 28 Weeks Later, offers more details about the origins of the Rage virus. Two London scientists attempting to develop an inhibitor to control aggressive impulses in humans, determined that the best way to distribute this inhibitor was via a contagion. One scientist genetically modified the Ebola virus to carry the inhibitor, but the virus mutated and reversed the inhibitor's effects. As such, instead of working to suppress rage, it had the opposite effect, stimulating rage instead, and thus creating the Rage virus. After the scientists are forced to kill the first human test subject when he becomes out of control, they cover up the incident by burying his body in a field in the middle of the night. Subsequently, the two scientists have a furious argument and one quits. Later that night, he tips off the animal liberation group about the experiments, before shooting himself in the head. The other scientist becomes infected by a primate along with the members of the liberation group. An animated version of first chapter of 28 Days Later: The Aftermath is available on both the 28 Days Later limited edition DVD and the 28 Weeks Later DVD. The animation can also be watched here, and its IMDb record is here.

Whilst never clarified in the film itself, it has been explained in the graphic novel, 28 Days Later: The Aftermath, that the infected target their victims through smell. Uninfected individuals smell of perfume, deodorants, soap, etc., while those that are infected would reek of perspiration and dirt. One could conclude that the virus would be counter-productive if an infected individual attacked another infected individual, ergo it has evolved to exclude that possibility. On a more practical level, one could argue that this facet of the Infected is based simply upon the tradition of the earlier zombie films that influenced 28 Days Later, insofar as in such films, zombies are never seen to attack other zombies.

Yes and no. As Jim is wrestling with the boy, it can be clearly heard on the soundtrack that the boy shouts "I hate you!" several times before Jim kills him with his baseball bat. In the director/writer commentary on the DVD, Danny Boyle discusses how the origin of the infection was "rage", so the filmmakers decided to layer a lot of "violent" speaking onto the soundtrack for scenes involving the Infected. According to Boyle, however, for this particular scene, they made it a little bit too loud. So, we could interpret the utterance as the attacking Infected's voice or as the film's Chorus.

Although it appears that Frank and Hannah were surviving just fine, with Frank having found a way to protect them from the infected entering the tower, there was one factor that hampered their ability to survive in the building...lack of fresh water. When Jim is shaving his face he is cutting himself quite a bit, and Frank tells him that they can't spare water for shaving. Also, when Frank takes Jim up to the roof of the building where he and Hannah have laid out hundreds of containers to catch rainwater, he tells Jim that rain has been very scarce since the outbreak.

The Tunnel would have most likely been sealed by the English and French government shortly before or after the Infection first became a pandemic, as sealing off the Tunnel would prevent the Infection spreading to mainland Europe. If not, there is the risk that, no matter how small their numbers, a few Infected could get into the Tunnel by chasing after fleeing trains and cars. They could then run towards France, and survive long enough to infect a maintenance worker or military personnel assigned to guard the Tunnel. If any of these mainland countries have outbreaks of the Infection that are not controlled, all of Europe, and eventually Asia and Africa, could likewise become threatened by the Infection. Consequently, it's highly likely that the authorities-that-be sealed the Tunnel at the first indication of a pandemic infection.

The fighter jet pilot says, "Lähetätkö helikopterin?" (Finnish for "Could you send a helicopter?"), an indication that Jim, Selena and Hannah were seen this time.

Technically, there have been, at one time or another, 8 possible endings to the film, all of which are included, in way, shape or form, on the DVD.

1. "Jim Dreams and Dies": The original scripted ending (which is not the ending that exists in the finished film) involved an extension of the hospital scene after Jim, Selena, and Hannah have escaped the mansion. In this version, the scenes of Selena working on Jim are much longer, and are intercut with a kind of impressionistic "flashback" to the accident which put Jim in hospital prior to the opening of the movie. After doing everything they can to save Jim's life, Selena and Hannah eventually realize that he is gone, and they despondently leave the hospital together. This ending was shot and edited, and was the original planned ending for the film, but initial test audiences responded negatively, prompting director Danny Boyle to conclude that because the film was so bleak and had asked so much of the audience prior to the dénouement, to end it on such a low note was simply too harsh. Boyle was also worried because he felt the audience misinterpreted the final shot (Selena and Hannah walking out of the hospital, with the doors swinging closed behind them), and he didn't like this level of ambiguity. Specifically, Boyle and scriptwriter Alex Garland had always intended the last shot to signify that Selena and Hannah were going to survive no matter what happened, but test audiences took it to imply they were heading off to certain death. This ending can be found on the DVD in the deleted scenes, under the title "Hospital Dream".

2. "Jim Dies": The same as above, but without the flashback scenes. This was the ending found at the end of the credits on American prints of the movie and when it was played on Sky Movies in the UK.

3. "Rescue Coda": The theatrical ending with Jim surviving the gunshot wound and he, Selena and Hannah hiding out in the mountain district until they are found and rescued. This is the version with which the theatrical release and the DVD release end.

4. "Rescue Coda without Jim": A combination of the "Jim Dies" ending and the "Rescue Coda" ending. Jim dies in the hospital, but rather than the film end with Selena and Hannah leaving, it cuts to the cottage and the rescue scene simply plays out without Jim. This can be found on the DVD in the Alternate Endings under the title "Alternate Ending".

5. "Escape Ending": During shooting of the film, for a period of time, Boyle planned to end the movie with the shot of the car driving away from the mansion after Mailer (Marvin Campbell) has dragged Major West through the back window. This was because the production had run out of money, and simply couldn't afford to shoot anything else (the film was shot almost entirely in sequence). After the Fox executives saw the movie with this ending however, they agreed to provide more money to shoot both the "Jim Dies" endings and the "Rescue Coda" endings.

6. "Freeze-Frame Ending": Boyle briefly toyed with ending the movie on the freeze-frame as the taxi slams through the gates of the mansion. Fox didn't like that ending, so they gave Boyle extra money to shot the ending in the Lake District.

7. "Radical Alternative Ending": This ending was never shot, but it was storyboarded. See here for details. It can be found as a series of animated storyboards in the Alternate Endings on the DVD narrated by Danny Boyle and Alex Garland.

8. "Alternate Theatrical Ending": This version is similar to the "Jim Dreams and Dies" ending, but with some minor differences; before they drive through the gate, as Jim is lying on the backseat of the car, he tells Selena that he saw a plane through the trees, thus giving her hope for the future. This ending is included on the Blu-ray disc.

...Jim walks around deserted London: East Hastings by Godspeed You! Black Emperor.

...Jim enters the church and find the corpses: The Church by John Murphy.

...Jim, Selena and Mark make their way to Jim's parents' house: Abide With Me, written by Henry F. Lyte and William H. Monk. The version heard in the film is sung by Perri Alleyne and arranged by the movie's composer, John Murphy.

...Jim and Selena are chased by the Infected in the Tower Block: Tower Block by John Murphy.

...Jim, Selena, Frank and Hannah leave the flat: Taxi (Ave Maria), composed by Charles Gounod. The version heard in the film is sung by Perri Alleyne and arranged by John Murphy.

...Jim, Selena, Frank and Hannah is chased by the Infected in the tunnel: The Tunnel by John Murphy.

...in the shopping scene: AM 180 by Grandaddy.

...Jim, Selena, Frank and Hannah approach the roadblock outside Manchester: In Paradisium, composed by John Murphy. If it sounds familiar, it could be because you heard it in the opening scene of The Thin Red Line (1998) (1998).

...Frank dies: Frank's Death - Soldiers (Requiem in D Minor) by John Murphy.

...Jim sees the plane on the sky: Then There Were Two by John Murphy.

...during the climax: In the House - In a Heartbeat by John Murphy.

28 Days Later: Original Soundtrack Album (XL-Records) (2002): 1. "The Beginning" (2:56), 2. "Rage" (1:22), 3. "The Church" (1:16), 4. "Jim's Parents (Abide With Me)" (2:29), 5. "Then There Were" 2 (0:42), 6. "Tower Block" (1:26), 7. "Taxi (Ave Maria)" (2:09), 8. "The Tunnel" (1:40), 9. "A.M. 180" (by Grandaddy) (3:20), 10. "An Ending / Ascent" (by Brian Eno) (4:27), 11. "No More Films" (0:48), 12. "Jim's Dream" (0:40), 13. "In Paradisum" (by Gabriel Fauré) (2:11), 14. "Frank's Death / Soldiers, Requiem in D Minor" (2:39), 15. "I Promised Them Women" (1:24), 16. "The Search for Jim" (2:41), 17. "Red Dresses" (0:48), 18. "In the House - In a Heartbeat" (4:17), 19. "The End" (1:55), 20. "Season Song" (by Blue States) (4:12), 21. "End Credits" (1:48), and 22. "Taxi / Ave Maria Remix"* (by Jacknife Lee) (6:14).

* = US release only

Unless otherwise stated, all music by John Murphy.

Yes. Both the US edition, released in 2007, and the UK edition, released in 2008, are identical to the original DVD releases. Note that the film was also released in the UK in 2013 in a Limited Edition Steelbook version. This version has no additional special features.