Travis Bickle, an honorably discharged U.S. Marine, is a lonely and depressed man in New York City. He becomes a taxi driver to cope with chronic insomnia, driving passengers every night around the boroughs of New York City. He also spends time in seedy porn theaters and keeps a diary. Travis becomes infatuated with Betsy, a campaign volunteer for Senator and presidential candidate Charles Palantine. After watching her interact with fellow worker Tom through her window, Travis enters to volunteer as a pretext to talk to her, and takes her out for coffee. On a later date, he takes her to see a Swedish sex education film, which offends her, and she goes home alone. His attempts at reconciliation by sending flowers are rebuffed, so he berates her at the campaign office, before being kicked out by Tom.
Travis confides in fellow taxi driver Wizard about his thoughts, which are beginning to turn violent, but Wizard assures him that he will be fine. Disgusted by the street crime and prostitution that he witnesses throughout the city, Travis finds an outlet for his frustration and begins a program of intense physical training. He buys guns from dealer Easy Andy and constructs a sleeve gun to attach on his arm, with which he practices drawing his weapons. One night, Travis enters a convenience store moments before a man attempts to rob it, and he shoots the robber. The shop owner takes responsibility and Travis leaves. On another night, teenage prostitute Iris enters Travis's cab, attempting to escape her pimp Matthew "Sport" Higgins. Sport drags Iris from the cab and throws Travis a crumpled twenty-dollar bill, which continually reminds him of her. Some time later, Travis hires Iris, but instead of having sex with her, attempts to dissuade her from continuing in prostitution. He fails to completely turn her from her course, but she does agree to meet with him for breakfast the next day, and Travis becomes obsessed with helping her return to her parents' home. Travis leaves a letter to Iris at his apartment saying he will soon be dead, and inside the letter, money for her to return home.
After shaving his head into a mohawk, Travis attends a public rally, where he attempts to assassinate Senator Palantine, but Secret Service agents notice him and he flees without taking a shot. He returns to his apartment and then drives to the East Village, where he confronts Sport. Travis shoots Sport, then walks into Iris's brothel and shoots off the bouncer's fingers. After a wounded Sport shoots Travis, grazing his neck, Travis shoots and kills him. Iris's client, a mobster, appears and shoots Travis in the arm, but Travis reveals his sleeve gun and kills the gangster. The bouncer continues to harass Travis, causing Travis to stab him in the hand and shoot him in the head to kill him. As a horrified Iris cries, Travis attempts suicide but, out of ammunition, resigns himself to a sofa until police arrive. When they do, he places his index finger against his temple gesturing the act of shooting himself. While recuperating, Travis receives a letter from Iris's parents, who thank him for saving her, and the media hail him as a hero. Travis then returns to his job and encounters Betsy as a fare. She discusses his newfound fame, but he denies being a hero and drops her at her place free of charge. He glances at her in his rear view mirror as he drives away.