In New York City, Travis Bickle takes a job as a night shift taxi driver to cope with his chronic insomnia and loneliness. He frequently goes to the porn theaters on 42nd Street and keeps 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 in the city and dreams about ridding "the scum off the streets."

Travis becomes infatuated with Betsy, a campaign volunteer for Senator and presidential candidate Charles Palantine. Travis enters the campaign office where she works and asks her out for coffee, to which she agrees. Betsy confesses that she feels a special connection to Travis and agrees to go on another date with him. During their date, Travis takes Betsy to a porn theater, which repulses her and causes her to leave. He attempts to reconcile with her, but to no avail. Enraged, he storms into the campaign office where she works and berates her before he is ordered to leave.

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. In an attempt to find an outlet for his rage, Travis begins a program of intense physical training. A fellow taxi driver recommends him to a black market gun dealer, Easy Andy, from whom Travis buys four handguns. At home, Travis practices drawing his weapons, and modifies one to allow him to hide and quickly deploy it from his sleeve. He begins attending Palantine's rallies to scope out his security. One night, Travis shoots and kills a man attempting to rob a convenience store run by a friend of his.

On his trips around the city, Travis regularly encounters Iris, a 12-year-old child prostitute. He solicits her and tries to persuade her to stop prostituting herself. Soon after, Travis cuts his hair into a mohawk and attends a public rally where he plans to assassinate Palantine. However, he is chased away by Secret Service agents, who see him unzipping his jacket and putting his hand inside. That evening, Travis drives to the brothel where Iris works to shoot her pimp, Sport. He enters the building and engages in a shootout with Sport and one of Iris's clients, a mafioso. Travis is shot several times, but manages to kill the two men. He then brawls with the bouncer, whom he manages to stab through the hand with his knife located in his shoe and finish off with a gunshot to the head. Travis attempts to commit suicide, but is out of bullets. Severely injured, he slumps on a couch next to a sobbing Iris. As police respond to the scene, a delirious Travis imitates shooting himself in the head using his finger.

Travis goes into a coma due to his injuries. He is heralded by the press as a heroic vigilante and is not prosecuted for the murders, also receiving a letter from Iris's parents, thanking him. After recovering, Travis grows his hair out and returns to work, where he encounters Betsy as a fare; they interact cordially, with Betsy saying she followed his story in the newspapers. Travis drops her at home, and declines to take her money, driving off with a smile. He suddenly becomes agitated after noticing something in his rearview mirror, but continues driving into the night.