Travis Bickle (Robert De Niro) is revolted by what he considers the moral decay around him. One night while on shift, Iris (Jodie Foster), a 12 year-old child prostitute, gets in his cab, attempting to escape her pimp. Shocked by the occurrence Bickle fails to drive off and the pimp, "Sport" (Harvey Keitel), reaches the cab. Sport gives Bickle a crumpled twenty dollar bill, which haunts Travis with the memory of his failure to help. Later seeing Iris on the street he pays for her time, although he does not have sex with her and instead tries to convince her to leave this way of life behind. The next day, they meet for breakfast and Bickle becomes obsessed with saving this naive child-woman who thinks hanging out with hookers, pimps and drug dealers is more "hip" than dating young boys and going to school.

Any lingering doubt in the viewer's mind about Bickle's insanity is obliterated when he is suddenly and shockingly shown to be sporting a crude Mohawk haircut at a public rally in which he actually attempts to assassinate Senator Palantine. He is spotted by Secret Service men and flees. Bickle returns to his apartment, then drives to Alphabet City where he shoots Sport, before storming into the brothel and killing the bouncer, wounded Sport (who has followed Bickle), and Iris's mafioso customer. He then calmly tries repeatedly to fire a bullet into his own brain but all the weapons are empty so he resigns himself to resting on a convenient sofa until police arrive on the scene of mayhem and carnage. Bickle survives this "heroic" mission in spite of himself and there is much more black irony to follow.

(excerpt from Wikipedia)