An attempt to genetically re-engineer the measles virus to cure cancer becomes lethal, infecting 99% of the world's population, killing the vast majority and turning those it does not kill into vampiric, albino, cannibalistic mutants called Darkseekers, who are extremely vulnerable to sunlight and roam at night: preying on the scant few who are naturally immune.
Three years after the outbreak, in 2012,[8] U.S. Army virologist LTC Robert Neville lives an isolated life in deserted Manhattan, unsure if any other immune humans are left. Neville's daily routine includes experimenting on infected rats to find a cure for the virus, searching for food and supplies, and waiting each day for any survivors who might respond to his continuous recorded radio broadcasts, which instruct them to meet him at midday at the South Street Seaport. Flashbacks reveal his wife, Zoe, and daughter, Marley, died in a helicopter accident during the chaotic evacuation of Manhattan as the military was enforcing a quarantine of the island. Neville himself stayed behind with other military personnel. Neville's only companion is his German Shepherd Samantha (Sam) and to cope with his loneliness, he regularly "talks" to some mannequins and the characters on film recordings from video stores. At night, he barricades himself with Sam inside his heavily fortified Washington Square Park home to hide from the Darkseekers.
One day as Neville hunts a deer, Sam pursues it into a dark building. Neville cautiously goes in after her and locates the deer's corpse along with Sam, but discovers the building is infested with Darkseekers. Both escape unharmed and the attacking Darkseekers are killed by the sunlight. Neville finds a promising treatment derived from his own blood, so he sets a snare trap and captures a female Darkseeker. A male Darkseeker attempts to pursue them but is halted by the sunlight and returns to the shadows. Back in his laboratory in the basement of his house, Neville treats the female seemingly without success.
The next day, Neville notices "Fred", a mannequin usually at the local video store, positioned outside Grand Central Terminal and shoots it in confusion. However, Neville realizes he is being watched by the Darkseekers from nearby buildings. As he approaches Fred, he is ensnared in a trap similar to the one he used to capture the female and is rendered unconscious after hitting his head. By the time he wakes up the sun is setting and he is attacked by infected dogs. Neville and Sam kill them, but Sam is bitten during the fight (dogs are immune to the airborne strain of the virus but can be infected with a different strain through bites). Neville injects her with a strand of his serum, but when she shows signs of infection, Neville is forced to strangle her to death as she begins to turn. Heartbroken and driven by rage, Neville ventures out and deliberately attacks a group of Darkseekers the following night, before he is rescued by a pair of immune humans, Anna and a young boy named Ethan, who have traveled from Maryland after hearing his broadcast.
They transport the injured Neville back to his home, where Anna explains they survived the outbreak aboard a Red Cross evacuation ship from São Paulo, and are making their way to a survivors' camp in Bethel, Vermont. Neville argues no such survivors' camp exists. As he continues working to cure the female Darkseeker, Neville theorizes by lowering her body temperature with ice, he can increase the treatment's potency. The next night, a group of Darkseekers, who tracked Anna and Neville the night before, invade the house. Neville, Anna, and Ethan retreat into the basement laboratory sealing themselves in with the female test subject.
Theatrical ending
As the Darkseekers attack the lab, discovering the last treatment was successful, Neville assesses the situation as the Darkseeker alpha male rams himself against a glass door to break in. Neville draws a vial of blood from the woman he cured, and gives it to Anna, before shutting Ethan and her inside a coal chute in the back of the lab. Neville then kills both himself and the attacking Darkseekers with a grenade, saving the cure.
The following day, Anna and Ethan arrive at the survivors' camp in Bethel, where they are greeted by military officers and other survivors before Anna hands them the cure. Anna narrates how Neville's efforts and sacrifices to save humanity ultimately became legend.
Alternate ending
As the Darkseekers attack the lab, the alpha male Darkseeker creates a butterfly shape while attempting to break through the glass to the laboratory. Neville realizes it is referencing the butterfly-shaped tattoo on the female Darkseeker's neck and the alpha male and his followers have simply been trying to recover his mate. Neville puts his gun down and returns the female. Neville and the alpha male stare each other down and Neville apologizes after seeing the latter emotionally respond to his mate's return. The alpha male departs with the rest of the pack. Once they are gone, a shocked Neville looks over at the many pictures of his test subjects and realizes he has become a monster in the eyes of the infected, showing remorse for the experiments he has undertaken over the years.
The next morning, Neville abandons his research and heads along with Anna and Ethan to Vermont as a changed man, in the hope of finding the survivors' colony. They cross the George Washington Bridge, while Anna delivers a hopeful monologue ending with the statement: "You are not alone."
While initially released as a book-accurate alternate ending on the two-disc special edition DVD of I Am Legend in 2008, the "alternate" ending was removed from the theatrical cut due to negative response from initial test screening audiences.[9][10][11][12][13][14] In February 2023, it was confirmed that the sequel to I Am Legend would consider the "alternate" ending to be the canonical one, with Will Smith reprising his role as Dr. Robert Neville, as he resumes his experiments on the Darkseekers.[15]
