Moderately successful criminal defense attorney Mickey Haller (Matthew McConaughey) operates around Los Angeles County out of a Lincoln Town Car driven by a former client working off his legal fees (hence the title). Haller has spent most of his career defending garden-variety criminals, until he lands the case of his career: Louis Roulet (Ryan Phillippe), a Beverly Hills playboy and son of real estate mogul Mary Windsor (Frances Fisher), who is accused of the brutal beating of a prostitute. But the seemingly straightforward case suddenly develops into a deadly game of survival for Haller.Roulet is seemingly innocent and was at the wrong place at the wrong time. Haller and his investigator Frank Levin (William H. Macy) analyze the pictures and evidence, notably the injuries the victim sustained. It bears a similarity to a past case of Haller's that landed a previous client, Jesús Martínez (Michael Peña), in jail for murdering a woman, despite always proclaiming his innocence.Haller spends time with his ex-wife, prosecutor Maggie McPherson (Marisa Tomei), who has never appreciated Haller's efforts on behalf of guilty criminals when he ought to be trying to put them behind bars. All his clients claim to be innocent, but Haller begins to wonder if he should have tried harder on behalf of Martinez instead of convincing him to plead guilty in exchange for avoiding the death penalty.In prison, Martínez becomes agitated when Haller shows him Roulet's picture. Haller begins to suspect that Roulet could be the real killer in the Martínez case, but bound by attorney-client confidentiality rules, he cannot tell the police what he has learned.Levin is mysteriously killed after leaving a voicemail message claiming that he has found Martínez's ticket out of jail. Haller is suspected of killing Levin because a collector's gun missing from his house was used to kill Levin, a gun that Haller believes was taken by Roulet after the latter broke into Haller's home.Obliged to do his best for his client, guilty or not, Haller ruthlessly cross-examines the beaten prostitute and discredits her in the jury's eyes. After a prison informant lies to the prosecution on the witness stand, the defense moves to dismiss all charges in the current case. Roulet is set free, delighting his mother, but the police then arrest Roulet immediately for the previous murder case, based upon testimony Haller coaxed out of the witness.Haller acquires a gun from his driver, Earl (Laurence Mason), as a precaution against any retribution he may face. Roulet is released due to lack of evidence and sets out immediately to kill Haller's wife and child, but Haller finds out in time to get them out of the house. He is waiting as Roulet arrives and draws his gun. Roulet mockingly tells Haller he won't be able to guard his family this way every day. But a group of bikers that Haller has previously represented brutally assaults Roulet.Upon arriving home, Haller discovers Roulet's mother inside. She shoots him with his own gun, the same one that killed Frank, confessing that she committed that murder. Haller, wounded, kills her with his new gun.After being discharged from the hospital, Haller discovers that Frank had found a parking ticket issued to Roulet near the house of the murdered victim, which constitutes strong evidence against Roulet in his pending murder trial. Martínez will be released, and Haller rides off to his next case.