The protagonist is Bob Arctor, member of a household of drug users, who is also living a parallel life as Agent Fred, an undercover police agent assigned to spy on Arctor's household. Arctor/Fred shields his identity from those in the drug subculture and from the police. (The requirement that narcotics agents remain anonymous, to avoid collusion and other forms of corruption, becomes a critical plot point late in the book.) While posing as a drug user, Arctor becomes addicted to "Substance D" (also referred to as "Slow Death", "Death" or "D"), a powerful psychoactive drug. A conflict is Arctor's love for Donna, a drug dealer, through whom he intends to identify high-level dealers of Substance D. Arctor's use of the drug causes the two hemispheres of his brain to function independently or "compete". Through a series of drug and psychological tests, Arctor's superiors at work discover that his addiction has made him incapable of performing his job as a narcotics agent. Donna takes Arctor to "New-Path", a rehabilitation clinic, just as Arctor begins to experience the symptoms of Substance D withdrawal. It is revealed that Donna has been a narcotics agent all along, working as part of a police operation to infiltrate New-Path and determine its funding source. Without his knowledge, Arctor has been selected to penetrate the organization.