Detroit P.D. Detectives Frank Agnew and Joe Geddes drown fellow cop Brendan McCann in a restaurant sink. "I'm not a bad person," Frank says. "No, but he is," Geddes replies. They cuff Brendan's corpse to the steering wheel of his car and send the car into the river in an attempt to make his death look like a suicide.
Damon Callis shoots drug dealer Billy Hobson at a trap house and steals a cocaine stash belonging to mob boss Alexander Skelos. Damon and his wife, Maya, worry that Skelos will discover his treachery and punish him.
Internal Affairs officer Simon Boyd reveals that I.A. had been building a case against Brendan. Boyd asks Frank to walk him through Brendan's home security camera footage from the night of the murder, in which Frank and Geddes staged dropping off a drunk Brendan at home to corroborate their story.
Brendan's car is recovered. In the trunk, police find the body of Anton Bobek, an I.A. informant against Brendan. During a police search of Brendan's house, Frank discovers a prepaid mobile phone with a voicemail from his girlfriend, Katia. "Hi Brendan, it's Katia. Yes," she says. He listens to the voicemail repeatedly, then keeps the phone.
Damon secures a location for a blind pig. Before opening for business, he offers a bag of cocaine to Reverend Lowdown as a gesture that he will not infringe on the Reverend's drug-dealing territory.
At Boyd's request, the coroner examines the water in Brendan's lungs and reports that Brendan did not drown in the river.
Geddes tells Frank that Brendan used Katia to lure Bobek to his death. He says he spared Katia's life and dropped her off in Windsor, Canada, telling Frank not to look for her for their own protection as well as hers. A flashback reveals that Geddes likewise ordered Katia never to contact Frank. Frank disregards Geddes' warning and looks for Katia in Windsor, but is unable to find her.
Frank and Geddes interrogate Skelos, who employed Bobek. Skelos hints that Geddes is in his pocket. Dani later brings in a witness, Raymond LeFevre, who saw two men send Brendan's car into the river. Frank and Geddes subject LeFevre to an interrogation so disorienting, he ends up uncertain about what he saw.
Boyd learns that Bobek was last seen with an Eastern European hooker and tracks down Katia's old address.
Dani is assigned to investigate Billy's death and talks to Billy's mother, Roslyn, and Billy's friend, Daryl Singers. Meanwhile, Geddes questions Bobek's associates and learns that Billy and Bobek were friends. Dani, Geddes and Frank visit the trap house where Billy was shot, and Geddes suggests that Brendan was drowned in the kitchen sink.
Maya is visited at the bar by Sean, her ex-husband and an ex-cop. Damon throws him out. High on coke, Sean later visits Frank, his former partner, at home.
Pressured by the mayor's office to come up with developments in the case to pass on to the press, Frank and Geddes spend all night brainstorming ways to pin Brendan's murder on Billy. The next day they present their scenario to the mayor, suggesting Brendan and Bobek were killed by Billy, who was later killed by an accomplice over a drug-related rift. The resulting newspaper headline, "Detective's Homicide Linked to Drug Slaying," alarms Damon, who assumes Skelos is framing him for Brendan's murder. Meanwhile, Dani is skeptical of the story.
Poppa T turns on Damon for letting Reverend Lowdown believe they were operating the blind pig with Skelos' blessing. A shootout ensues and Michael is killed. On Damon's orders, Nick later shoots Poppa T dead. Damon confesses his schemes to Skelos and asks for protection against the Reverend, threatening to rat Skelos out. Skelos refuses, and instead teams up with the Reverend to take over the blind pig and The International.
Frank traces Katia to an Internet sex site and starts an online chat with her. She shuts down the connection once she realizes it's Frank, but he enlists a source to trace her location to Chicago. He pays her a surprise visit, but she turns him away. Disheartened, Frank visits Dani at home and sleeps with her.
Frank reaches out to Reverend Lowdown for leads on Brendan's murder. "Drop a name, any heat on you drops with it," Frank tells him. The Reverend suggests Frank look into Damon. Meanwhile, Dani finds security camera footage of Damon and Michael at a corner store near the trap house on the day of Billy's murder.
Following the Reverend's lead, Frank arrests Damon in connection with Billy's murder. He and Geddes plot to frame Damon for the murders of Billy, Brendan and Bobek, though Damon's crew tries to pin the murders on Michael. After getting a phone call from Skelos, Geddes steers the case away from Damon, who's on the verge of ratting out Skelos. His about-face angers Lt. Dawson and results in a two-week suspension. Damon is released.
Frank later confronts Geddes about conspiring with Brendan to declare murders as suicides to cover for Skelos. Geddes then asks Frank about his trip to Chicago, revealing that he knows Frank is looking for Katia.
Dani reluctantly tells Boyd that she thinks Geddes killed Brendan but refuses to believe Frank is involved. While wearing a wire, she gets Frank to talk about his relationship with Katia and learns that Katia was a prostitute, leading Boyd to surmise that she may be the Eastern European prostitute seen with Bobek. Boyd requests a surveillance warrant for Frank and Geddes.
Frank aces his testimony in the murder trial of Lady Belle, a case he solved with Geddes. Meanwhile, Geddes goes to Chicago and kills Katia by pushing her off a balcony. Realizing that Geddes tracked Katia down, Frank rushes to Chicago to save her but arrives too late.
Damon plots to kill Skelos before Skelos kills him, even after Skelos threatens to harm Maya's children if Damon makes a move against him. Maya refuses to support Damon's plot against Skelos and walks out on him.
Geddes admits to Lt. Dawson that he and Brendan falsely declared murders as suicides but implicates Lt. Dawson for signing off on the reports. He says they can protect themselves if Lt. Dawson stops Dani from investigating Damon.
Unhinged by Katia's death, Frank condemns Lt. Dawson for overseeing a corrupt department. "And you murdered a cop," Lt. Dawson hits back. Frank tells Geddes he's turning himself in, then calls several families to tell them that a couple of dirty cops falsified their loved ones' murders to make them look like suicides.
Frank cashes out his bank account and books a flight to Germany, but after missing his plane, he carjacks a businesswoman's car and drives to his ex-wife's house in Ann Arbor, where he contemplates suicide. Local police escort him back to Detroit, where he learns that Sean, who read all of Frank's case files while staying at Frank's house, has confessed to the murders of Brendan, Bobek and Billy.
Refusing to let Sean take the fall, Frank enlists Maya to act as Sean's alibi. However, Sean refuses every attempt from Frank to clear his name. "I'm no good," he tells Frank. Out of options, Frank links Brendan's prepaid phone to Sean and plants it in the evidence locker.
Boyd formally accuses Frank and Geddes of killing Brendan and presents a compelling case to the Deputy Mayor and Assistant Prosecuting Attorney. However, Frank privately convinces the A.P.A. that their hard-fought Lady Belle case will fall apart if his name gets tarnished by Boyd's allegations. Boyd is furious when his accusations are dismissed.
Damon and Nick shoot Skelos and dispose of his body at an auto junkyard. They go to Flint to lay low, but Reverend Lowdown's men find them and gun Damon down.
Frank claims Katia's body at the coroner's office, but is unable to provide an answer when the corner asks for the name of the deceased.