Michael Clayton (George Clooney) is a "fixer" for a prestigious New York City law firm, using his connections and knowledge of legal loopholes for clients' benefit. After leaving an underground poker game and dealing with a wealthy client's (Denis O'Hare) hit and run, Michael drives despondently and stops at a remote field, where some horses are standing on a hill. As he climbs the hill to admire them, the car explodes behind him.
The story moves back to four days earlier. Michael is $75,000 in debt from a restaurant investment he entered with his brother Timmy, which collapsed when Timmy used the restaurant's funds to fuel his drug habits. Michael, having gone to a loan shark to raise the money, refuses to give up his brother and is held responsible for the debt, given one week to come up with the money. Meanwhile, one of the firm's leading attorneys, Arthur Edens (Tom Wilkinson), has a manic episode in the middle of a deposition in Milwaukee involving a class action lawsuit against U-North, an agricultural products conglomerate. Michael arrives in Milwaukee and bails Arthur, who had failed to take his medication, out of jail, but Arthur escapes from their hotel room in the middle of the night.
Karen Crowder (Tilda Swinton), U-North's general counsel, discovers that Arthur had come into possession of a confidential U-North document detailing the company's decision to manufacture a weed killer it knew to be carcinogenic. Karen brings this to the attention of U-North's CEO Don Jeffries (Ken Howard), who puts her in contact with two men secretly on retainer (Robert Prescott and Terry Serpico). She contracts them to follow Arthur and bug his apartment and phone. When they report that Arthur is building a case to expose his own client, Karen approves of their taking drastic measures. The men murder Arthur in such a way as to make it look like an accidental drug overdose or suicide.
Michael, saddened by Arthur's death, becomes suspicious upon learning that U-North was planning a settlement just a few days before and that Arthur had booked a flight to New York for one of the plaintiffs, Anna (Merritt Wever). She tells him that no one knew of her conversations with Arthur, not even her attorney, yet Michael's firm knew of Arthur's conversations with the U-North plaintiffs. With the help of his other brother Gene (Sean Cullen), a police detective, Michael gets access to Arthur's sealed apartment and, finding a bottle of champagne and two glasses in the refrigerator, suspects Arthur arranged to meet Anna. There is also a copy of Realm and Conquest, a fantasy novel which Michael's son Henry recommended to Arthur. He finds a receipt from a copy store being used as a bookmark.
Michael is caught in the apartment and arrested for trespassing, but Gene bails him out. Using the receipt, Michael discovers that Arthur had ordered three thousand copies of the confidential U-North document. Michael takes one with him; the two hit men (who are now following him) inform Karen. Michael is about to show his boss Marty Bach (Sydney Pollack, in his penultimate film appearance) what he has discovered, only to be offered a renewal of his employment contract as well as an $80,000 bonus he had requested to cover his debt. An additional confidentiality agreement causes Michael to realize Marty already knows about U-North's shady dealings.
One of the hit men rigs Michael's car with a bomb, but is forced to leave it unfinished when Michael returns. While at a card game, Michael receives a phone call summoning him to meet the hit-and-run client in Westchester County, as seen at the start of the movie. He is being followed by the two men, who have trouble trailing him, but eventually get close enough to detonate the remote bomb. However, Michael had left the car to admire the horses in the field, since it resembled an illustration from Realm and Conquest. Michael throws his personal effects into the fire and escapes into the woods.
At a U-North board meeting at a Manhattan hotel, Karen proposes approval of a new settlement agreement in the class action lawsuit against the company. She leaves the room to let the board of directors deliberate, and is confronted by Michael in the hotel foyer, who informs her he has access to the U-North memo and knows about her role in Arthur's murder, and the subsequent attempt on his life. He goads Karen into offering him $10 million for his silence. Karen reluctantly agrees, prompting Michael to reveal a phone in his pocket that has conveyed their conversation to the police. Jeffries comes out of the board meeting to check on her. Alarmed by Michael's presence, he calls out for security, but police suddenly arrive and arrest him and Karen instead. Michael hands the phone to Gene, then leaves the building, hails a cab, passes the driver 50 dollars and tells him to "give me 50 dollars' worth. Just drive."
Who did Michael enter into the restaurant investment?
His brother