Johnny Basil is an undercover narcotics officer, working under the false identity of Jamaican drug addict "Desmond Mobay", who has been sent into the Oswald Maximum Security Penitentiary ("Oz") to investigate the drug trade. Basil gets too deep undercover, and in the process of attempting to prove his loyalty to the gangsters, he becomes a genuine heroin addict and kills an incarcerated police officer. Later, after being confronted by Augustus Hill, he confesses to the murder. He serves time in Oz's police unit, Unit J, where he is eventually killed by Clayton Hughes, an incarcerated former correctional officer.
Basil arrives prepared to join with the Homeboys, the African-American gang within "Emerald City", Oz's experimental cell block. He gets a job as an assistant to Warden Leo Glynn, keeping him informed and having free access to phones and computers; with these, he regularly contacts his partner, Nancy Mears, and his supervisor about what is happening within Oz. Glynn is initially the only person in Oz who knows of Basil's true identity, and he warns him about a similar operation which ended in the murder of the undercover cop, Paul Markstrom. Glynn also warns Basil that he is not allowed to use drugs. Basil tells Glynn that undercover narcs have ways of faking drug addiction.
Inside Em City, Basil rooms with Augustus Hill, who tells him that if he wants drugs, he should go to Kenny Wangler. When Wangler and Junior Pierce die in a shooting inside Em City, Basil asks inmate Arnold "Poet" Jackson if he can join the Homeboys. Homeboy leader Simon Adebisi is suspicious of Basil, and asks him for the name of a person who can vouch for his criminality. Basil has planned a potential parolee at Lardner named Nester Parks to vouch for him, in exchange for having an easy parole hearing. Nancy visits Basil regularly, posing as his girlfriend. Hill recognizes her from the night of his arrest, but does not say anything until later on. Basil realizes this and tells Nancy, who denies encountering Hill, not to visit for a while.
As his first test, the other inmates ask him to snort heroin to prove that he is one of them. He does so and ends up in solitary confinement during a drug shakedown. He is fully addicted by the time he gets out, but Adebisi does not yet see him as worthy enough for the Homeboys. His second test involves being able to take a punch from Chucky Pancamo, the leader of the Italians. Basil is severely beaten, but stays standing long enough to gain some of the gangs' respect. Poet gives him some heroin as comfort for his wounds, which he uses immediately. Basil's incoherent e-mails cause Nancy to suspect that he is using drugs and should be replaced. However, he convinces her and Glynn both that he is not using and that he is close to busting Adebisi, Pancamo, and other inmates.
The gang leaders vote in a straw poll over whether or not Basil is worthy of membership. Pancamo votes yes; Adebisi votes no; and Latino leader Enrique Morales abstains. To break the tie, his third test is to kill an inmate and make it look like an accident. Meanwhile, Bruno Goergen, a racist cop who sent to Oz for killing a weapons dealer, blackmails Basil for protection in exchange for keeping his undercover identity secret. Deciding him to be "Mobay's" murder victim, he lures Goergen to an open elevator shaft, telling him that Hill has found out he is a cop and wishes to expose him. As Goergen and Hill struggle, Basil pretends to help Goergen but then pushes him into the shaft, killing him. This solidifies the trust of the two uncertain gang leaders, and he is granted membership.
A new unit manager, Martin Querns, allows Adebisi, Pancamo, and Morales to do as they wish in Em City as long as there is no violence. Drugs are sold in the open and Basil is pressured to entice "babies" (new customers) to buy. Basil uses money he brought into Oz to pretend to find his "babies", while using the heroin he buys. The gangsters decide that Basil must sell drugs in front of them to prove that he is one of them, leading Glynn to suggest that another undercover detective should pose as a customer so there are no complications regarding the sale of drugs by a police officer. The cop, Cecil Brand, is seen purchasing drugs from Basil and Poet, and Pancamo assures Adebisi that Basil is still worthy of membership.
Detective Guinevere McGorry questions inmates about Goergen's death. "Mobay" agrees to help if he finds sufficient information. Adebisi uncovers Brand's identity and commands Basil to kill him. Basil is prepares to do so until he is confronted by Hill, who calls him a fraud for breaking the law in the name of the law. Angry, Basil threatens and assaults Hill. Shamed, Basil confesses to McGorry and Glynn that he murdered Goergen because he wanted to prove that he is not a fraud, but real. He is tried for murder and sent to Unit J for at least three years, the unit for incarcerated police. As a result of Basil's arrest, the drug investigation is over.
In Unit J, Basil rooms with former Oz corrections officer Clayton Hughes and a sheriff named Alvin Yood. Glynn urges Basil to visit his wife, his lieutenant, and his former partner Nancy. He apologizes to his lieutenant, who is sorry to lose him to prison, and to Nancy, who is angered by his presence and says she is not sorry that he's in prison for what he did. Basil also meets his wife, whom he is scared to see since he let her down. After her visit, Basil insults Hughes, who in turn murders him. Hughes is sent to solitary as a result. Basil's body is later seen in the infirmary.
- Bruno Goergen - Murdered by Basil, pushing him on the bottom of the elevator shaft because he was threatening to put word in Oz that Basil was an undercover cop, and also so Basil gets in good with Enrique Morales, Chucky Pancamo & Simon Adebisi if he killed someone in Oz so he continues to bust the drug trade in Oz.