Sherman McCoy (Tom Hanks) is a Wall Street investor who makes millions while enjoying the good life and the sexual favors of Maria Ruskin (Melanie Griffith), a Southern belle gold digger. Sherman and Maria are driving back to Maria's apartment from JFK Airport when they take a wrong turn on the expressway and the two find themselves in the "war-zone" of the South Bronx. They are approached by two suspicious black youths after Sherman gets out of the car to move a tire placed purposely in the middle of the road. Sherman jumps back into the car and Maria guns the engine in reverse, running over one of the teenagers and putting him in a coma. The two drive away and decide not to report the accident to the police.
Meanwhile, indigent alcoholic journalist Peter Fallow (Bruce Willis), anxious for a story to make good with his editor, comes upon the hit-and-run case as a rallying point for the black community calling upon Jewish district attorney Abe Weiss (F. Murray Abraham), who is the Bronx District Attorney seeking re-election. According to Judge Leonard White (Morgan Freeman), almost all of DA Weiss' prosecutions end up with black and Puerto Rican defendants going to prison and Weiss is seeking a white defendant for purposes of convincing the minority-majority community that he is worth re-electing.
Weiss recognizes the press coverage inherent in prosecuting the callow Sherman, who has been discovered as the hit-and-run driver, in order to cultivate the image as an avenger for the minorities and be propelled to the mayorship of New York City. As Sherman is brought to his knees, New York City fragments into different factions who use the case to suit their own cynical purposes.
Finally, Sherman is left without any allies to support him except for the sympathetic Judge Leonard White and the remorseful Fallow. Fallow gains a tremendous advantage and insight into the case when he is dating a woman who is the sub-letting landlady of Maria's apartment, and knows of secret recordings of conversations in the apartment made by the authorities to prove that the woman is not in fact living in the rent-controlled apartment herself. She discovers information about the McCoy case (where Maria states she was driving the car), which she gives to Fallow, who in turn covertly supplies it to Sherman McCoy's defense lawyer.
Sherman gets his hands on a tape and plays the recording in court, where it reveals Maria directly contradicting the evidence she has just given, showing she has been perjuring herself and causing her to faint. Sherman plays the tape in a tape recorder inside his briefcase connected to a small loudspeaker that he holds on the desk.
When the judge orders that he approach the bench with this evidence, he asserts that the tape is all his (making it admissible evidence and it is technically truthful since it refers only to the dummy tape he was holding and ignores the real tape that is hidden which is not his), resulting in his acquittal.
The people in the court go into an uproar, to which Judge White launches into a tirade that they have no right to act self-righteous and smarmy, or that they are above Sherman, considering Reverend Bacon (John Hancock) claims to help disadvantaged New Yorkers but actually engages in race baiting, or that the District Attorney Weiss pushed this case not in the interest of justice but in the interest of appealing to minority voters to further his political career by appealing to their desire to "get even". After the Judge made his point, he begs the people to be decent and change their ways, letting Sherman go.
The film ends as it begins, where there is a large audience applauding Peter Fallow's premiere of his book. Fallow says that Sherman McCoy has moved away from New York City to an unknown destination, presumably to live in obscurity.

Question: Who secretly supplied the information about the McCoy case  to Sherman's lawyer?
Answer: Fallow
Alternative answer: A woman that Fallow is dating