Diary of the Dead is the fifth film in Romero's highly acclaimed Dead series of zombie films. It is not a direct sequel to previous films in the series, and "a rejigging of the myth" according to Romero.

Diary of the Dead follows a band of people making a horror film at the time of the first outbreak who decide to record the epidemic incident documentary-style and end up themselves being chased down by zombies. It appears to be set in a present time (though it takes place in the same time frame as the original).

The movie begins with an introduction to a documentary called "The Death of Death". The movie then shows a view from a cameraman and a news reporter, reporting on the murder of a local family. The reporter and newswoman chat while the ambulance crew load the deceased bodies. Suddenly, one of the bodybags move and an ambulance worker attempts to resuscitate the body when he is bitten. Another zombie approaches the news crew and police open fire on them but are ineffective at killing the zombies. The news lady is bitten on the neck while the cameraman films. The scene cuts to a group of young film studies students from the University of Pittsburgh: Jason, Ridley, Francine, Mary, Debra, Tony, Tracy, Gordo and Eliot, who are in the woods making their own horror film about a mummy along with their faculty advisor, Andrew Maxwell, when they hear on the radio the news of apparent mass-rioting and mass murder. Soon, these reports include cannibalism and the fact that the recently deceased are inexplicably waking and walking.

Ridley and Francine decide to leave the group, while Jason, who wants to film the events documentary-style, heads to the dormitory of his girlfriend Debra Monahan. When she cannot contact her family they travel in Mary's RV to the house of Debra's parents in Scranton, Pennsylvania.

While driving her RV, Mary sees a car accident and runs over a highway patrolman and three other zombies trying to escape from them. The group stops and, after running over an undead state trooper, and then having to endure Tony declaring that she just killed real, living people, Mary tries to commit suicide shooting herself in the head with a pistol. Her friends take her to a hospital where they realize that the dead are returning to life as zombies and they need to fight to survive while traveling to Debra's parents' house. Mary reanimates and is shot in the head by Professor Maxwell. On their way out, Gordo is bitten by a zombie patient which is then killed by Eliot. After Gordo dies, he is shot by Tracy as he reanimates

On their way the fuel line of the RV is cut but they are assisted by a deaf Amish man named Samuel. The real surprise here is Tracy, who reveals that she is familiar with auto repair because of her father's profession. The barn they are hiding in is surrounded by zombies whom Samuel fights off as the RV is repaired. Just before escaping, Samuel is bitten and takes his own life by stabbing himself with a scythe through his head, but also taking the zombie who bit him with him.

They later pass a city and are stopped by an armed gang. They try to barter with the men, who are better armed and supplied. The men are survivors of the attack on their city; everyone else left. The leader is a member of the National Guard, and does not believe the military will help. They rest in the city for a while, and edit their videos. There is a small argument amongst Jason and Debra, who does not share his belief in the need for videos. Jason sees the recordings as a necessity to show the world what was really happening. His uploaded videos reach 72,000 hits in eight minutes. They also find out that Debra's family were camping for the extent of the crisis, and is just now on their way home.

The National Guard survivors gear up after one of their members has a heart attack, dies, then goes missing. They mistakenly kill one of their own and then chase the zombie into the fuel supply area, killing it with a canister of hydrochloric acid. The students resupply and leave for Debra's house. They get a video stream from Tokyo from a survivor until they lose signal. The radio towers have become unreliable. The only dependable source of information is from the internet, added by bloggers.

When they arrive at Debra's house they assume that her family are still missing but find the reanimated corpses of her mother and brother who they fight off. They escape the house and decide to travel back to Ridley's mansion. Members of the real National Guard stop and rob them. The leader of the unit enters the RV and forces Jason at point blank to turn off his camera while they rob the RV. They steal all but the weapons (the bow, a handgun and a shotgun) and run off shooting their rifles in the air as they head to an island as their next destination.

The group watches video feed from a zombie hunting group. The group enter a building and find an old couple, who warn them not to come in. The group enters anyway and one of their members is attacked by a zombie. They kill three of them then the three uninjured men debate over whether or not to kill the couple for not telling them about their family members. The injured member kills the old couple before taking his own life.