A lonely and alienated Vietnam War veteran works the night shift as a taxi driver on the gritty streets of New York City. Struggling with insomnia and an intense disdain for the perceived depravity around him, he becomes obsessed with the idea of purifying the city. His chronic isolation and desperation for human connection propel him into a cycle of encounters with the city's inhabitants, from the affluent campaign worker to whom he's drawn, to the streetwise young prostitute whom he feels compelled to save. Musing about the filth around him and the need for a cleansing rain to wash the scum off the sidewalks, he becomes caught in the web of his own volatile disaffection.

As his grip on reality weakens, he seeks out an arsenal and begins a rigorous physical training regime, articulating an increasing conviction that he is destined for a grand, if vague, violent confrontation. His interactions grow stranger and his judgments more erratic, the taxi driver starts plotting an assassination, targeting a prominent politician. However, an unforeseen event during his preparation shifts his focus, and he turns his attention more intently on rescuing the adolescent prostitute from her circumstances, seeing it as a path to redeem both her and the rest of the city from the decay he so passionately despises.