Swann is a "cave cop," a member of the New York City Transit Police, who guard the citizens underground. It is the night of the Statue of Liberty centennial, when most of the city is down by the harbor watching the biggest fireworks display in American history, and Swann is posted in the subway station beneath Times Square. As the returning celebrants pour through the station, Swann spots a gang of teenage thieves.

He follows them out of the subway and into the street and ends up wrestling on the sidewalk with one of them, fighting for his own gun. When he hears the sound of a shot, Swann discovers with surprise that he has killed the teenager. The shooting is declared justified, but as Swann returns to his old routine, he realizes that his view of life will never be the same again.

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Against the glittering background of New York City, Swann finds a new self. When the Money Store shows him the path to instant fortune, Danica, the green-eyed girl of his high school fantasies, finds him suddenly intriguing, and together they begin an adventure involving stretch limousines, elegant restaurants, trendy nightclubs, and intimate nights at the Plaza Hotel. Coleman, a tiny black plainclothes policewoman who dyes her pigtail police blue, introduces him to another side of New York life.

Then there is Swann's beleaguered wife, Ellen, a champion coupon redeemer; his father, Joseph, who listens to World War II combat sounds on tape; Lieutenant Gentile, a police investigator who shows abnormal curiosity about Swann's newfound wealth; and dozens more characters caught up in the drama of Michael Daly's Under Ground.