Tom Ludlow, an LAPD undercover detective, has spiraled into alcoholism following his wife’s death. Working in the elite Vice-Special unit, he orchestrates a sting operation against a Korean gang involved in human trafficking. To gain their trust, he allows them to steal his car and a machine gun, unaware that he has planted a tracking device inside the vehicle. With their location exposed, Ludlow storms their hideout, eliminating the gangsters and rescuing two kidnapped girls. Though his unit praises his actions, he faces immediate scrutiny from his former partner, Detective Terrence Washington. Tired of corruption, Washington has begun feeding information to Captain James Biggs of Internal Affairs.
Believing Washington has betrayed him, Ludlow tracks him to a convenience store with the intent of confronting him. Before he can act, two masked gunmen burst in under the guise of a robbery and assassinate Washington. In the chaos, Ludlow engages in a shootout, but surveillance footage captures him appearing to shoot Washington. With the video as evidence, his actions are indefensible. Captain Jack Wander, his commanding officer, intervenes, instructing him to erase the footage. He provides Ludlow with an alibi, stating that he arrived too late to save Washington.
The investigation identifies criminals Fremont and Coates as Washington’s killers, further complicated by the discovery of a large sum of cash in Washington’s possession. Internal Affairs concludes that Washington was corrupt, stealing drugs from evidence and selling them. However, Ludlow refuses to accept this version of events and recruits Detective Paul Diskant, the officer handling the forensic evidence, to uncover the truth.
Following leads, they track down a secluded property in the hills, where they find the decomposing bodies of the real Fremont and Coates, revealing they were killed long before Washington’s murder. Ludlow visits Washington’s widow, giving her the surveillance footage, and swears to avenge her husband’s death.
Ludlow and Diskant devise a plan to expose the truth by posing as corrupt cops willing to take over the drug trade. With the help of an informant, Scribble, they arrange a meeting with the criminals posing as Fremont and Coates. When Diskant recognizes them as impostors, he is shot and mortally wounded. Ludlow swiftly retaliates, killing both men and fleeing to his girlfriend’s house. There, he sees a news report identifying the criminals as undercover Los Angeles County Sheriff’s Department deputies. Branded as a cop-killer, Ludlow realizes the conspiracy extends deeper than he anticipated.
As he processes this revelation, Detectives Cosmo Santos and Dante Demille break in and subdue him. They confess to orchestrating the murder of Washington and framing him by planting evidence. Ludlow realizes that Washington had been informing on Wander, who was running a drug theft operation from the department’s evidence room.
Santos and Demille transport Ludlow to the remote site where Fremont and Coates were buried, planning to execute him. In a desperate fight for survival, Ludlow overpowers and kills both corrupt detectives. He then rushes to Washington’s home, where he finds Sergeant Mike Clady searching for the surveillance footage. Ludlow subdues Clady, preventing him from harming Washington’s widow, and locks him in the trunk of his car.
Determined to end the conspiracy, Ludlow confronts Wander at his home. Following a brutal fight, he subdues the corrupt captain, who confesses to having incriminating evidence on high-ranking officials, with ambitions of using it to climb the ranks to LAPD chief and, eventually, mayor. Wander tries to bribe Ludlow, but he refuses and executes him.
Shortly after, Captain Biggs and Sergeant Green arrive. Biggs reveals that they orchestrated events to use Ludlow as a means to dismantle Wander’s operation. Offering Ludlow an alibi, Biggs hints that the department still has a place for him, leaving Ludlow standing in the wake of his moral reckoning.