The movie follows a single day in the life of a young LAPD officer named Jake Hoyt (Hawke) as he is subjected to a 24 hour evaluation by Alonzo Harris (Washington), a highly decorated detective within the LAPD narcotics division.
It quickly becomes apparent that Jake's 'by the book' dictum is in stark contrast to Alonzo's philosophy of blending in with the street. They first detain some college students buying marijuana from a dealer, but instead of arresting them they merely confiscate the drugs.
Jake is put into a compromising position when Alonzo offers him marijuana as a test of his street smarts, putting a gun to his head and threatening to throw him out if he doesn't smoke it. Unknown to Jake, the marijuana is laced with PCP). Jake relents and smokes, taking three hits from the pipe.
Alonzo then takes Jake to the home of a drug dealer and bookmaker "Roger" (Scott Glenn), with whom Alonzo seems to have a close relationship with. As they're cruising down the street later on, Jake notices a girl (Samantha Esteban) being attacked by two criminals in a side alley. Jake jumps out of the car and saves her, while Alonzo watches. Jake wants to arrest the men and get a statement from the girl, but Alonzo dismisses her and leaves the two men out on the street to face 'street justice', though not before intimidating them. Jake discovers the girl's orange wallet before he leaves and picks it up, whereupon he realizes that the girl is 14 years old.
During the rest of the day, Alonzo brings Jake further and further into the world of the 'street' as he harasses a drug dealer named Blue (Snoop Dogg) and learns about another drug dealer named Sandman. He then illegally searches Sandman's home and steals thousands of dollars in the presence of Sandman's wife (Macy Gray) and nephew. Sandman's wife then calls for help when she realizes the money was stolen, the local gang members which appeared to be Crips down the street start to take action, by shooting Alonzo's car on which he then fires back. He then takes Jake to the Jungles to meet his Salvadoran mistress, Sara (Eva Mendes), and their young son, where he establishes that the gang members in the housing project all fear and respect him. During their entry to the housing project, Jake and Alonzo see a flock of pigeons controlled by a resident, and Alonzo explains to Jake that the residents use the pigeons to warn the community the police are present. Alonzo then meets with three high ranking law enforcement officials (a D.A., an LAPD Captain and a high ranking Detective) — known as the "Three Wise Men" (Tom Berenger, Harris Yulin, Raymond J. Barry) — where it is clear from their conversation that Alonzo has bigger problems than breaking in a new rookie. Alonzo receives permission from the wisemen to "cash in" on an "account."
Alonzo goes back to Roger's home with Jake and some fellow narcs, they seizes Roger's money stash hidden underneath the floor of his kitchen, offering a cut to the team (Jake refuses his share). Alonzo then takes Jake's shotgun and shoots Roger as he sits unarmed and helpless. Jake is horrified by what he had just witnessed and while the crew sets out to manipulate the crime scene, he snaps, resulting in a tense standoff between him and the corrupt officers. But he soon realizes his predicament and surrenders when Alonzo mentions the department's blood test as his wild card, which would ruin him as it would detect the PCP he had smoked earlier that day.
The backup arrives to clean up, and Jake expresses his disgust at the way Alonzo operates, to which Alonzo replies that it is part of his methodology; he had to cozy up to Roger in order to take him out, which sums up his pragmatic view of "law enforcement", but Jake believes otherwise.
The pair later arrive at the home of Chicano gangster "Smiley," (Cliff Curtis) who is playing poker with two other gang members: "Sniper" (Raymond Cruz) and "Moreno" (Noel Gugliemi). After playing a hand, Jake becomes aware that Alonzo has abandoned him to the thugs, and Smiley informs him of Alonzo's situation. Jake tries to escape, but the trio quickly overpowers him and drags him into the bathroom to be shot. They ignore his pleas for mercy and search his pockets, finding the orange wallet he had picked up earlier which happens to belong to Smiley's cousin. Smiley immediately recognizes the wallet and demands to know how Jake acquired it. Jake desperately tells him where he found it. Not believing him, Smiley calls his cousin, who confirms Jake's story and provides his physical description. In appreciation for Jake's brave actions, Smiley spares Jake's life, returns his pistol and allows him to leave.
Jake returns to Sara's apartment looking for Alonzo. He attempts to arrest him as he tries to make his appointment with the Mafia members, but Alonzo resists. Jake eventually subdues him, after which gang members and local residents begin congregating to watch the conflict. Alonzo tries to get the crowd on his side, but it becomes evident that the neighborhood has had enough of him. They allow Jake to walk away with the money that he intends to turn in as evidence. Detained by one of the gang members, an enraged Alonzo then taunts and challenges the defiant mob, screaming" I'm the police, I run shit here! You just live here!" In turn, they abandon him in disgust.
Alonzo is seen driving when he is ambushed by several vehicles and executed by the very Mafia members he was going to pay.
The final scene has Jake pulling into his driveway and going home to his family, while a radio broadcast reports Alonzo's death. Ironically, the news report of Alonzo's death is a facsimile of a line spoken earlier in the movie by Alonzo himself, which was used to persuade Jake to take part in Roger's murder/robbery: "An LAPD narcotics officer was killed today serving a high-risk warrant, near LAX. An LAPD spokesperson said, officer Alonzo Harris was survived by his wife and four sons."
Fuqua wanted Training Day to look as authentic as possible, and he shot on location in some of the most infamous neighborhoods of Los Angeles, California. He even obtained permission to shoot in the Imperial Courts housing project. According to Fuqua's commentary on the DVD release of the film, the actors and crew ended up receiving a warm welcome from local residents. When Fuqua wasn't able to shoot a scene directly on location, he recreated these locations on sets.
There were two police officers on hand as technical advisors, Michael Patterson and Paul Lozada. Cle Shaheed "Bone" Sloan served as the gang advisor. Washington, Hawke and other cast members also met with undercover police officers, local drug dealers and gang members to help them understand their roles better.
The film was released in theaters on October 5, 2001, and was a box office hit, landing at #1. At its second week of release, the films gross revenue was $13,386,457, landing again in the #1 position. The film stayed in the top-ten box office until the seventh week of release, landing at #12. The film grossed $76,631,907 domestically and $104,876,233 worldwide.