Travis Bickle is a strange, anonymous taxi driver, drifting through the streets of New York at night, hanging out in porno theaters, and privately raging against the depravity of the city in his diary. We learn little about his background, including the fact that he's apparently an ex-marine and a Vietnam veteran. Part of him is yearning for love, or some kind of connection. He seeks it with Betsy—a worker for the campaign of presidential candidate Charles Palantine.
Initially, Betsy seems interested, too, and agrees to go on a date with him. Things go terribly wrong when Travis decides to take her to a porno movie.
Yeah, he's that oblivious.
Betsy is turned off and leaves. Travis grows increasingly angry...and increasingly unstable. The other taxi drivers don't seem capable of giving him any good advice.
Gradually, Travis delves deeper into violent fantasies. He starts working out, trying to turn himself into a first-rate killing machine, and buys an arsenal of guns. At one point, he shoots a robber who's trying to stick-up a convenience store; it's unclear whether Travis actually killed him or not, though its certainly possible. At the same time, he gets obsessed with liberating a twelve-year-old prostitute named Iris, who seems sick of working for her pimp, an oily guy named Sport. He meets her when she tries to get into his cab, before Sport drags her back out.
Eventually, Travis starts fantasizing about murdering the presidential candidate Betsy works for. When he makes his move, however, he can't get a clear view of Palantine, and the Secret Service spots him. He escapes successfully without attracting any arrest warrants.
Instead of killing Palantine, then, he decides to vent his rage in a more socially acceptable manner: Travis goes on a rampage, killing Iris' pimp, Sport, and two other gangsters who are in the brothel/hotel where Iris works. After his murder spree, Travis tries to kill himself...but he's out of ammo.
Surprisingly, he isn't sent to prison. Instead, the New York newspapers hail him as a hero for rescuing Iris and returning her to her parents. Against all apparent odds, his rage and madness prove to be what the city was looking for.
But by the movie's end, it's unclear whether Travis' madness and rage will resurge and take control of his life once more. It's also unclear whether or not the whole "hero" thing is just a product of Travis' fevered, uber-volatile imagination.