In New York City, former Marine and Vietnam War veteran Travis Bickle takes a job as a night-shift taxi driver to cope with his chronic insomnia and loneliness, frequently visiting adult movie theaters on his breaks, and keeping a diary in which he consciously attempts to include aphorisms such as "you're only as healthy as you feel". He becomes disgusted with the crime and urban decay that he witnesses regularly in the city and dreams about getting "the scum off the streets", recording these dreams in his diary. His narration throughout the film is largely conveyed through voice-over excerpts from his diary, which reveal his deteriorating mental state and deepening alienation from society.
Travis becomes infatuated with Betsy, a campaign worker for Senator Charles Palantine, who is running for president. Travis enters the campaign office where she works and asks her to join him for coffee, to which she agrees. Betsy agrees to go on a second date with him, during which he takes her to an adult movie theater, which she leaves immediately. He attempts to reconcile with her, but fails. The rejection further deepens his isolation and contributes to his increasingly erratic behavior. Enraged, he storms into the campaign office where she works and berates her before being kicked out of the office.
Experiencing an existential crisis and seeing various acts of prostitution throughout the city, Travis confides in a fellow taxi driver, nicknamed Wizard, about his violent thoughts. However, Wizard dismisses them and assures him that he will be fine. Despite this reassurance, Travis remains emotionally unmoored, unable to articulate the depth of his psychological distress. To find an outlet for his rage, Travis follows an intense physical training regimen. He gets in contact with black market gun dealer Easy Andy and buys four handguns. At home, Travis practices drawing his new weapons, going as far as creating a quick-draw firearm hidden in one of the sleeves in his coat. He begins attending Palantine's rallies to scope out his security. He also writes a threatening letter to Palantine, further indicating his intent to act against him. One night, Travis shoots a man attempting to rob a convenience store run by his friend, leaving before the cops arrive as the convenience store owner proceeds to beat the non-responsive robber.
In his trips around the city, Travis regularly encounters Iris, a 12-year-old child prostitute. Tricking her pimp and abusive lover Sport into thinking that he wants to solicit her, Travis meets with her in private and tries to persuade her to stop prostituting herself.
Travis shaves his hair into a mohawk and attends a public rally where he plans to assassinate Palantine. However, Secret Service agents see Travis putting his hand inside his jacket and approach him, which escalates to a chase. Travis escapes pursuit and makes it back to his home undetected.