Michael Clayton, a “fixer” for the prestigious New York law firm Kenner, Bach and Leeden, is in the midst of a poker game when the firm calls him to clean up the “mess” caused by Greer, one of their wealthy clients, who drove away from a hit-and-run accident. Leaving the game, Michael drives to Greer’s Westchester home, where he listens as Greer blames the victim for the accident, then abusively insults Michael when he advises that Greer needs a criminal attorney. Filled with disgust and self-loathing over his job as the company “janitor,” Michael drives into the countryside and pulls over when he sees three horses grazing on a ridge. As Michael gets out of his car and walks toward the horses, his car suddenly explodes and bursts into flames: Four days earlier, Michael learns that he must pay $75,000 to underworld figures within a week as a result of a debt he has incurred while trying to start a bar with his alcoholic, irresponsible brother Timmy. Upon returning to his office, Michael discovers that his close friend, star litigator Arthur Edens, has suffered a mental breakdown in Milwaukee while taking depositions in a class action suit against the firm’s largest client, the agrochemical giant U/North. Michael immediately flies to Milwaukee to contain the damage created when Arthur stripped naked in the deposition room and ran into the parking lot following one of the plaintiffs, Anna Kaiserson. In Milwaukee, Arthur, who has suffered from manic-depression in the past and now gone off his medication, rants to Michael that U/North is spewing poisonous carcinogens into the earth. Upon meeting Michael, U/North’s chief counsel, Karen Crowder, who has supplanted her lack of self-confidence with a venomous desire to succeed, wonders aloud why a lowly attorney who has spent seventeen years at Kenner, Bach and Leeden, yet has not been deemed worthy of a partnership, was sent to handle the situation, and reminds him that her company stands to lose three billion dollars if the plaintiffs win their case. Later, Karen, holding a report she found in Arthur’s stolen briefcase documenting that U/North knew that the pesticides it was producing were contaminating the drinking water of small farms, phones Mr. Verne, a covert operative at U/North, to suppress the evidence. On the morning that Michael is to fly back to New York with Arthur, Arthur slips out of the hotel and disappears. Back in New York, Marty Bach, the head of the firm, holds Michael responsible for Arthur’s disappearance. Searching for Arthur, Michael calls him at his loft, and when no one answers, leaves a message, which is heard by Verne and another operative, Mr. Iker, who have slipped into Arthur’s loft to bug his phone. Arthur, who is roaming the streets of New York, phones Anna at her family’s farm. Anna, who is sympathetic to Arthur, listens as he tells her that he has information that will benefit the 450 families in the class action suit. Meanwhile, Michael, broke and desperate to raise the $75,000, goes to Marty’s townhouse to ask for a loan. There Marty discloses that he has discovered Arthur is trying to build a case against U/North, and that if successful, he will bring down Kenner, Bach and Leeden. Marty then offers to give Michael the loan if he can control Arthur. Returning to the loft to look for Arthur, Michael is seen by Verne and his accomplice, who have the loft under surveillance. Spotting Arthur on the street, Michael warns him to stop trying to expose U/North, but Arthur refuses to listen, asserting that now Michael is his enemy, too. Soon after, Arthur leaves a message on Karen’s answering machine in which he reads the document proving that U/North and Don Jeffries, the head of the company, knew that they were poisoning wells with their pesticide. Upon hearing the message, Karen instructs Verne to eliminate Arthur. Soon after, Verne and Iker break into Arthur’s apartment and inject him with a lethal dose of drugs, making his death appear a suicide. Michael, who blames himself for Arthur’s death, is puzzled by the absence of a suicide note, and when he learns that U/North is settling the case, he phones Anna’s farm. Anna’s sister answers the phone and informs Michael that Anna flew to New York at Arthur’s behest, but that Arthur never turned up at the airport to meet her. Michael then drives to Anna’s motel before she can catch her flight back home. There Anna tells Michael that Arthur phoned her, asking her to come to New York because he wanted to show her a document that would win the class action suit. When Anna states that she told no one about the document, Michael realizes that Arthur’s phone was tapped. Michael then asks his brother, Gene, a New York police detective, for the key to Arthur’s loft, which has been sealed by the police. At the loft, while going through Arthur’s papers, Michael finds a receipt for a copy store and pockets it. Handing the receipt to the clerk at the copy shop, Michael is presented with boxes containing thousands of copies of the document proving U/North’s culpability. Grabbing a document, Michael fails to notice that Verne and Iker are standing behind him in line and once he leaves, they secure copies of the document for themselves. Michael hurries to the office to show Marty the document, only to discover that Marty knew about it, but chose to suppress it because it would mean a loss of $9,000,000 in fees from U/North, plus U/North was threatening to release the video of Arthur disrobing, a disclosure that would jeopardize the lucrative merger between Kenner, Bach and Leeden and a London firm. To insure Michael’s silence, Marty offers him a check for $80,000 on the condition that he sign a confidentiality agreement. Accepting the check, Michael pays off the debt on the bar. Michael then goes to a poker game, but is soon summoned to the Greer house. Leaving his poker game, Michael hurries to his car, which is parked on the street. Michael’s unexpected arrival interrupts Verne and Iker, who, following Karen's orders to eliminate Michael, are in the middle of planting a bomb and tracking device in his car. As a result, the tracking device is not functioning properly, making it difficult for the killers to follow Michael’s car and detonate the bomb. Just as Michael exits his car to look at the horses, the assassins find the tracking signal and detonate the bomb, unaware that Michael is no longer in the car. Realizing that someone is trying to kill him, Michael throws his identification into the flames to make it look as if he was incinerated in the car. At the U/North board of directors meeting, Karen recommends that the settlement proposal for the class action suit be confirmed, boasting that U/North can write off the entire settlement on its taxes. Leaving the conference room feeling triumphant, Karen is surprised by Michael, who confronts her with the incriminating document. Declaring that as a fixer, he is the one she should buy, not kill, Michael threatens to expose U/North unless the company pays him $10,000,000. When Karen agrees to transfer the money into his account, Michael reveals that he had a cell phone in his pocket and that the police have heard their entire conversation, then walks away as the police arrive to arrest Karen and Jeffries. On the street, Michael hails a taxi and hands the driver $50 to “just drive.”