I Am Legend is set in 2009, when a genetically-modified virus that is meant to cure cancer fails and kills in return 90% of the people infected by it. The remaining 10% turns into predacious and nocturnal creatures called “Darkseekers.” These creatures are extremely sensitive to sunlight and artificial UV rays. Three years after the outbreak, the US Army virologist Lieutenant Colonel Robert Neville (Will Smith) roams around the now deserted New York.
The virus has infected animals on varying degrees. Those who live near humans are infected but wild animals are not. The outbreak has caused ruined most establishments and the animals from wildlife parks roam the streets. Everyday, Neville tries to find a cure for the virus. From time to time, he goes to Manhattan to replenish his supplies. He also continuously play broadcasts calling for survivors. Neville’s companion is a German Shepherd named Sam. His entertainment comprises of mannequins and old videos and recordings. During night time, he secures himself and his dog inside his home in Washington Square Park from Darkseekers. An incident involving Sam and a deer leads to an attack by the Darkseekers. Fortunately, both manage to escape the deadly creatures.
Neville secures a possible treatment for the virus using his own blood. To test this, he captures a female darkseeker while a male one observes from afar. His tests prove to be futile when the virus poses no effects on the female. The day after, it is Neville who is captured in the same way that he trapped the female Darkseeker. He escapes but he is also attacked by infected dogs. Sam comes and helps Neville escape the rabid animals. After their escape, Neville finds out that Sam is bitten. He tries to treat the dog with his serum but to no avail. At the end, Neville strangles the dog when it start showing symptoms of infection. Sam’s death bring Neville despair. Blindly, he sets out to kill as many Darkseekers as he can until he is surprisingly saved by two survivors, Anna and Ethan. The two come all the way from Maryland after hearing his radio announcements. As they try to bring back Neville to his home and treat his wounds, Anna explains that there is a survivor’s camp in Bethel, Vermont and she and Ethan are headed there. Neville does not believe a word that Anna says about the survivor’s camp.
Once back, Neville tries to treat the female Darkseeker in his lab again. The next night, the trio are attacked by Darkseekers who had followed Anna and Ethan. The house is overrun by the creatures and the three of them lock themselves inside the lab with female Darkseeker. Inside the lab, Neville finally discovers that his treatment has become effective. He then draws blood from the cured female Darkseeker and gives a vial of it to Anna. Neville saves them by locking Anna and Ethan inside a coal chute in the back of the lab before blowing himself and the Darkseekers with a grenade.
An alternate ending shows an alpha male Darkseeker smudging something that looks like a butterfly on the glass. Neville recognizes that the male Darkseeker is the partner of the one he captured. He returns the female Darkseeker and apologizes to the alpha male creature. The alpha male and the other Darkseekers leave Neville alone. Neville realizes the importance of this discovery on his search for the cure. This alternate ending shows Neville, Anna and Ethan on their way to the survivor’s camp carrying the antidote.