Robert Neville is the last man on Earth who is not infected by the virus that turns everyone into a vampire-like creature. By day, Neville scavenges for food and supplies to ensure survival. However, at night, Neville must stay safe in his home and safe from the terrifying creatures that are outside.
One day, when Neville was out searching for supplies, he sees a woman who looks like she's alive in a field. Neville runs after her and speaks to her.
"A woman. Alive. In the daylight" (Matheson).
Neville invites the woman, Ruth, to go back to his house. The two of them grow closer and bond talking about the love ones the lost, Neville lost his wife and Ruth lost her husband. Neville asks if he could test her blood, just to make she isn't infected, and she reluctantly agrees.
When Neville gets the test results back, he learns that Ruth is infected. Before he could do anything about it, Ruth knocks him unconscious. He later learns Ruth is a spy from a race of vampires that have learned to look and function like normal humans, they call themselves the "New Society." That night, members of this society break into Neville's home and take him captive.
As prisoner to the vampires, Neville learns that he is supposed to get publicly executed. However, Ruth comes to see him and offers him a way out. She gives him suicide pills so he would not have to suffer at the hands of the vampires. Before he dies, Neville asks Ruth to try to keep a little bit of humanity in the New Society. Ruth agrees to try her best to and kisses him goodbye. Neville then takes the pills, which kill him instantly.
Don't--let it get--" He coughed up blood. "Don't let it get--too brutal. Too heartless.""What can I--" she started, then stopped. She smiled at him. "I'll try," she said (Matheson).