The protagonist is Bob Arctor, member of a household of drug users, who is also living a double life as an undercover police agent assigned to spy on Arctor's household. Arctor 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.