Working late at night in dangerous neighborhoods, his customers tend to include pimps, addicts and thieves. He is visibly disgusted by them, and begins fantasizing about "cleansing" such "filth" from the streets.
He becomes smitten with a woman, Betsy, who works in the local campaign office of presidential candidate, Senator Charles Palantine. He often watches Betsy from his cab, and finally enters the office with the pretense of wanting to support the candidate, and asks her out. They meet for coffee, and Betsy finds him odd but intriguing, and agrees to see him again. He takes her to a porn theater he frequents. She is appalled and refuses to see him again.
After Betsy rejects him, Bickle becomes increasingly paranoid and starts acting out his vigilante fantasies. He buys several guns and takes to carrying them secreted about his person—taped to his limbs, for example, or in hidden spring-loaded holsters. He practices a menacing, tough guy swagger in the mirror — "You talkin' to me?" — to use on whoever angers him. Eventually, he shaves his head into a Mohawk. He attends one of Palantine's speeches, apparently intent on shooting him. However, he draws the attention of Secret Service agents and flees.
He becomes obsessed with saving Iris Steensma, a 12-year-old prostitute he has seen on his route. He pays her pimp, "Sport", for her time, but is not interested in having sex with her, instead he tries to persuade her to leave prostitution and return home. Iris rebuffs him, only increasing his anger and resolve to take her away from a dangerous life. He shoots and kills Sport, but is wounded in the process. He then goes on a rampage through a brothel in which Iris is servicing a client. Bickle kills the client and a bouncer.
The newspapers hail Bickle as a hero for rescuing Iris. While in the hospital, he receives a letter from her parents, thanking him profusely for returning their daughter to them (she had been sent home after the police arrived and found out she was a runaway). After recovering, he sees Betsy, who tells Bickle that she read about him in the papers, and when she gets out and asks him how much the cab fee was, he smiles and drives away.