The first season of the show focuses on the story of a multi-generational family business that deals in illegal drugs and the efforts of the police to curb their trade.
The series begins with one of the key figures in the drug cartel using intimidation to clear one of his young dealers of a murder charge. He comes up against homicide detective Jimmy McNulty, a member of a joint homicide/narcotics unit created to investigate a single drug-dealing operation.
Venting his frustration that the gang is responsible for as many as ten unsolved murders, McNulty gets reassigned to a detail nobody wants.
While his superiors and co-workers just want the heat to die down so they can drop the investigation, McNulty and an unlikely ally, narcotics detective Shakima Greggs, commit themselves to use any means necessary, including heightened surveillance and sophisticated wiretaps, to bust the gang and its elusive leader.