Criminal defense attorney Mickey Haller (McConaughey) operates around Los Angeles County out of a Lincoln Town Car. Haller has spent most of his career defending garden-variety criminals, including a member of a local biker gang, until he lands the case of his career: Louis Roulet (Phillippe), a Beverly Hills playboy and son of real estate mogul Mary Windsor (Frances Fisher), is accused of the brutal beating of a prostitute.



Haller thinks Roulet is innocent, having simply been at the wrong place at the wrong time. Haller and his investigator Frank Levin (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, Jesus Martinez (Michael Peña), in prison for life for murdering a woman, despite always proclaiming his innocence.



Haller has a daughter with his ex-wife, prosecutor Maggie McPherson (Tomei), who has never appreciated Haller's efforts on behalf of guilty clients. 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.



Martinez becomes agitated when Haller visits him at San Quentin and shows him Roulet's picture. Haller begins to suspect that Roulet could be the real killer in the Martinez case, but bound by attorney-client confidentiality rules, he cannot tell the police what he has learned. That night, Roulet breaks into his house, nonchalantly admits to the murder that put Martinez in prison, and makes veiled threats toward Haller's family.



Levin is mysteriously killed after leaving a voicemail message claiming that he has found Martinez's ticket out of jail. Haller is suspected of killing Levin because he was shot with a gun missing from his house, 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 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, but the police then arrest him immediately for the previous murder case, based upon testimony Haller coaxed out of the informant.



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. Just then, a group of bikers that Haller has previously represented brutally beats Roulet while Haller looks the other way. He got a call from Maggie that there was a parking ticket issued to Roulet near the house of the murdered victim — strong evidence against Roulet in his pending murder trial — meaning that Martinez will be released.



Upon arriving home, Haller discovers Roulet's mother inside. She shoots him with his own gun, the same one that killed Levin, confessing that she committed that murder to protect her son. Haller, wounded, kills her with his new gun.



After being discharged from the hospital, it is learned that Martinez has been released and the District Attorney will seek the death penalty against Roulet. All the while, Haller rides off to his next case.