In Season One of the raw, realistic HBO drama series, a joint homicide/narcotics police detail is created to investigate a drug-dealing operation in the city of Baltimore.
During the trial of D'Angelo Barksdale, a mid-level dealer accused of murder, the prosecution's star witness recants her testimony, resulting in a not guilty verdict. After the trial, Detective James "Jimmy" McNulty explains to Judge Phelan how he suspects the Barksdale crew for a slew of related murders. When the judge contacts McNulty's superiors about this revelation, McNulty is taken to task for his indiscretion. Meanwhile, D'Angelo is free to return to work, but he soon discovers that he's been demoted.
Detectives McNulty and Bunk visit the city morgue to view the body of William Gant, the state's witness in D'Angelo Barksdale's ill-fated murder trial. McNulty is convinced that Gant was murdered by the Barksdale organization, to send a message.
In the fallout from the ruckus at the Towers — including a looming Grand Jury investigation — Lieutenant Daniels reluctantly covers for his detectives, saying he sent them to the projects in the middle of the night. "If I tell you I knew they were going, I screwed up," he says to Deputy Commissioner Burrell. "If I tell you I didn't, I get my officers in trouble...I screwed up." However, Daniels does insist that Prez be placed on desk duty. Major Valchek, Prez's father-in-law, says he owes Daniels for helping Prez, and promises Daniels two new cars and a surveillance van as well as some recording equipment to further the investigation.
Bodie manages to slip through security and escape from lockup. Meanwhile Herc and Carver continue their renegade ways by travelling to Marlboro to find Bodie and beat him into giving up information on the case. Once they get there, Herc has a change of attitude after meeting Bodie's grandmother. Furious at being robbed of his product during the "re-up", Avon Barksdale instructs Stinkum, Wee-Bey and Stringer to double the existing reward to anyone who eliminates Omar and his crew. When Bodie returns to the pit, he explains how no jail can hold him because he's too tough. He further challenges D'Angelo's mettle by implying that D'Angelo's weak. D'Angelo responds by telling how he killed one of Avon Barksdale's ex-girlfriends after she threatened to give Avon up to the police. AFter hearing this, Bodie becomes cautiously impressed. Sgt. Landsman tells Major Rawls that despite all the screw ups, McNulty's tenacity and obsession with getting the job done usually results in closed cases and that the Major should probably cut McNulty some slack. Detective Freamon discovers D'Angelo's pager number just when Greggs and McNulty figure out a cloned pager could be the advantage they need.
When Avon becomes more and more suspicious, he orders Wee-Bey to change phone lines in his apartment. He also promotes Stinkum to manage a new territory and he gets word that one of Omar's crew members has been "got". Stringer Bell warns D'Angelo that a snitch may be in his camp and D'Angelo should tighten up his crew by withholding their pay. While talking with Orlando, D'Angelo learns of Stinkum's promotion. The detectives get their affadavit approved for a cloned pager but they are puzzled when the numbers they receive are not regular Baltimore phone numbers. The detectives are then more stunned when seemingly inept Detective "Prez" figures out the code and has to show them how it works. McNulty and Greggs get a lead on Omar's van and shadow it to a cemetery where there, Omar makes an offer. McNulty prepares his house for his kids' weekend visit only when he goes to pick the kids up no one's home. Bunk gets the file on an old murder case that could be connected to D'Angelo. Herc and Carver catch up with Bodie and attempt to interrogate him only it backfires when Bodie rebukes them. When Wallace and Poot alert D'Angelo that they've found Brandon, D'Angelo pages Stringer, who in turn orders Brandon's execution.
Brandon's bloodied body is discovered in a lot that coincidentally is located next to Poot and Wallace's home. Wallace in turn, reports this to D'Angelo, and tells him about how seeing Brandon's boy has started to bother him. D'Angelo tells Wallace that he needs to get over it, but Wallace is still unsure about all he is involved in. Wallace gets even more unsettled about the situation after Avon rewards D'Angelo and him with extra cash for their parts in getting Brandon taken care of in the first place.
Greggs, Herc, Carver, and Syndor grab the runner who is holding the re-up stash for the pit. Now that the cops have the stash, a furious Avon begins to question D'Angelo about a possible snitch in the pit. D'Angelo says it can't be possible and he knows nothing of a snitch. Stringer tells D'Angelo and his crew they are to now use pay phones a block away and to not use the same phone twice. This in turn disconnects all the detective's wiretaps they fought so long to get.
McNulty has his sons play the game of "front and follow" with Stringer Bell as the target at a local market, and the two get Bell's license plate number. As a result, the two get Stringer's license plate number and McNulty couldn't be happier. Greggs and Carver bust a driver picking up a suspiciously large amount of cash from the Towers, only they are forced to return the money on orders of the Deputy Commissioner. Avon finds out about Orlando's inquiries about branching out into "dirty" business, and makes it clear to Orlando that the plan will never happen.
Freamon gets Syndor and Prez started on the Barksdale money trail. They discover the usual ties to dummy companies, but they also find large sums of political contributions. Herc and Carver bust Wee-Bey holding $22.000 but only $15,000 is reported. Lieutenant Daniels discovers the discrepancy and orders them to find the money before the morning roll call. Omar's mission takes him to Proposition Joe where he promptly offers another recently stolen Barksdale stash to Joe in exchange for Avon's pager number.
After being clean for three days straight, Bubbles gets some strong advice from another rformer addict named Walon. He also gets a promise of extra help from Detective Greggs. When Omar’s murder attempt on Avon Barksdale fails, Stringer lays down some new laws for the Barksdale crew. There will be no phone use, no more money runs, a new pager number and anyone that needs to talk to Avon has to go through Stringer first. They also send a word to Omar that a truce will be offered to him. As the team goes through their setup of one of Barksdale’s people, one of the officers gets shot.
While Greggs' life hangs in the balance, the officers deal with the shock in many different ways: Herc, Carver, and Syndor get back on the case after Freamon puts the incident into perspective; After seeing McNulty close to the breaking point, Rawls instructs him not to have any guilt over Gregg's shooting because he's not the one responsible; Daniels is ordered by the Deputy Commissioner to raid all the locations to appease the Commissioner's desire for "dope on the table."
After the Barksdale's supply gets confiscated in another bust, Stringer gives more new rules to the crew. Avon and Stringer meet with Levy to discuss the next steps, which include the elimination of any and all persons that can connect Barksdale to the drugs. Wallace goes back to the pit and asks to be let back in but Stringer has another plan for Wallace. Stringer then asks Bodie to take Wallace's old job.
In the season finale, Greggs finds the strength to finger "Little Man" as one of the men who shot her. Daniels and McNulty go to the feds with crushing evidence about some Baltimore politicians tied to Barksdale, thinking they finally have him. However, the Feds only want the politiicans and not Barksdale. D'Angelo can't swing a deal, and agrees to a possession charge and 20 years hard time after a talk with his mother. As the show concludes, business resumes in the pit with Bodie and Poot leading the way and using some of D'Angelo's wisdom and tactics.