When the trial of a mid-level drug dealer ends on an unexpected note, the police department faces the challenge of getting the charges to stick to a major drug organization, run by kingpin Avon Barksdale.
As the investigation gains steam, tensions rise in the ranks of the team tasked with the investigation as different perspectives and unfit members complicate their mission, especially in the wake of a shocking murder.
As the team deals with the fallout of Herc, Carver, and Prez's actions, they catch a break in their investigation of Avon. D'Angelo tries to cement his place in the organization.
Avon plots retaliation against Omar to send a message to everyone on the market. McNulty and his homicide partner look into a murder that D'Angelo may have been responsible for.
As Avon's paranoia grows, D'Angelo is warned that they might have a mole in their midst. The Barksdales get a lead on Omar's location. Prez redeems himself when he helps the team figure out the pager results.
As Avon gets his revenge on Omar, Wallace is disquieted by what happens. McNulty faces pressure to close the case. Rawls threatens to undo the narcotics team's work when he insists on pressing weak murder charges against D'Angelo.
As Rawls continues to put pressure on the team, Avon becomes convinced that he is being betrayed. Stringer orders his men to use a countermeasure that threatens the police investigation.
A chance encounter leaves McNulty with Stringer's license plate. Tension continues to rise within the team after they run afoul of someone with a lot of power.
As the team starts investigating Avon's possible connections to politicians, Omar continues with his plans to seek revenge on Avon. The team tries to turn someone close to D'Angelo against him.
In the wake of Omar's attempt to kill Avon, the latter tightens security and comes up with a plan to manipulate Omar. McNulty tries to establish contact with Wallace.
Reeling from what happened to Greggs, the team is forced to carry out a series of raids and attacks, but internal strife soon reveals that one of them is leaking information.
Wallace tries to win back his place in the pit. The team catches a break when they manage to put a camera in Avon's club. Avon and Stringer go hunting for the mole.
As D'Angelo turns on Avon in the wake of Wallace's murder, the team gets what they need to go after Avon. Daniels and McNulty face heat for the choices they made over the course of the investigation.