The novel's main character is Robert Neville, apparently the sole survivor of a pandemic the symptoms of which resemble vampirism. The author details Neville's daily life in Los Angeles, as he attempts to comprehend, research, and possibly cure the disease that killed mankind, and to which he is immune (Neville assumes this is because he was bitten by a vampire bat who was "infected". Because it was not a human, it did not kill Neville, instead, he became ill for a period of time). Neville's past is revealed through flashbacks, while his emotional struggle to cope with losing his humanity is dealt with by going about a daily routine.
Every day Neville prepares for nightly sieges from a vampire horde. Neville spends the daylight hours repairing his house: boarding windows, hanging garlic garlands, disposing of vampire corpses and gathering supplies. Once darkness falls, the infected come out of hiding and lay siege to Neville's house. They taunt him and attempt to entice him out — he recognizes one vampire as a former friend, Ben Cortman.
After bouts of depression and heavy drinking, Neville decides to find the cause of the disease. He obtains books and other research materials from a library, and through painstaking research he discovers the root of the vampiric disease: a strain of bacteria capable of infecting both deceased and living hosts. However, he does not realize that the living hosts (the infected) are still inherently human, even though they exhibit all the signs of vampirism.
Neville comes across the seemingly uninfected woman, Ruth, abroad in the daylight and captures her. After the initial shock of seeing another human wears off, Neville becomes suspicious of Ruth and is skeptical of her story. He also notices that she is strongly against the killing of the vampires — he feels that if her story of survival was true, she would have become hardened to their fate, nevertheless the two live in a wary state of tolerance....