The film chronicles a pivotal single day in the life of LAPD officer Jake Hoyt, played by Ethan Hawke, as he undergoes evaluation under the watchful eye of the experienced Detective Alonzo Harris, portrayed by Denzel Washington. A decorated narcotics officer, Alonzo has the potential to catapult Jake's career forward. As they ride together in Alonzo's vehicle, the duo encounters members of Mara Salvatrucha engaging in drug transactions at a local park. Rather than making arrests, Alonzo opts to confiscate the drugs and surprisingly urges Jake to take a hit of the marijuana.
Initially resistant, Jake finally acquiesces when Alonzo threatens him with a gun, arguing that refusal could put his life in jeopardy at the hands of angry street dealers. Jake reluctantly smokes the marijuana, only to soon discover that it’s laced with PCP. As they drive along, Jake spots a high school girl being assaulted in an alley. He leaps into action, bravely subduing the attackers; however, Alonzo dismisses the need to report the incident, suggesting that the girl’s gang-affiliated cousin will seek vengeance on the assailants instead. After finding the girl’s wallet on the ground, Jake pockets it, voicing his discomfort with Alonzo's approach of vigilante "street justice," but Alonzo brands him as naive.
Their day takes a darker turn as they visit the home of a drug dealer known as the Sandman, using a fabricated search warrant that serves merely as a ruse for Alonzo to seize drug money. Jake protests, fearing the potential deadly repercussions of their actions. Later, they visit Alonzo’s Salvadoran girlfriend Sara (played by Eva Mendes) and their young son, before Alonzo meets with a group of senior police known as the "Three Wise Men." They warn Alonzo about his debts to the Russian Mafia and suggest he vacate the area. Confident in his ability to manage the situation, Alonzo brushes aside their concerns, promising to avoid any scandal.
In a grim twist, Alonzo leads Jake and additional narcotic officers to the home of Roger, a former police officer turned drug dealer. Utilizing their warrant, they manage to confiscate millions in cash hidden in the floor. However, Jake's integrity prevents him from taking a portion of the money, raising suspicion among his colleagues. Alonzo then escalates the situation by shooting Roger in cold blood and manipulating the scene to claim it was justified, pushing Jake into a moral dilemma.
Amid a tense standoff, with both Alonzo and Jake pointing guns at one another, Alonzo reveals LAPD will conduct a blood test on Jake that will unveil the PCP he consumed earlier. In exchange for cooperation, Alonzo offers to alter the test results, which leads Jake to concede against his better judgment. Alonzo seizes the opportunity to dismiss Jake's idealistic view of law enforcement.
Eventually, Alonzo drives Jake to the abode of a Mexican gangster Smiley (Cliff Curtis), claiming to run an errand. A twist unfolds when Alonzo hands a package of cash over to Smiley, who counts it while waiting for a poker game to commence. Caught in the mix, Jake experiences a tense interaction with Smiley and his crew. Once Smiley learns Alonzo has betrayed Jake, he reveals the stakes: Alonzo owes $1 million to the Russian mob by midnight or face fatal consequences.
Having paid Smiley to execute Jake, the plot thickens. After a scuffle, Jake narrowly escapes what seems like certain death. In a twist of fate, Smiley spares Jake, honoring the bravery he displayed in saving his cousin. Returning to Sara's apartment in search of Alonzo, Jake attempts an arrest, culminating in a fierce altercation where he ultimately prevails. Local gang members, tired of Alonzo’s arrogance and manipulation, choose to let Jake leave with the cash.
In a dramatic climax, Alonzo finds himself cornered by members of the Russian Mafia as he attempts to flee. They eliminate him, concluding his story with a chilling reminder of the consequences of his corruption. The film wraps up with Jake pulling into his driveway to reunite with his wife, Lisa (Charlotte Ayanna), and daughter, while a radio broadcast falsely narrates Alonzo's end as a noble sacrifice during a high-risk warrant operation near LAX.
