DS Steve Arnott joins AC-12, an anti-corruption unit, after refusing to cover up a botched raid. His first assignment is DCI Tony Gates, a respected officer suspected of corruption. Gates is entangled in a cover-up involving his mistress, Jackie Laverty, who commits a hit-and-run.

The investigation into Gates intensifies as Laverty is murdered, and Gates is framed for her death. He is then blackmailed by criminals. Jackie Laverty is murdered, and DCI Tony Gates commits suicide to protect his family. The season ends with the shocking revelation that DC Dot Cottan is a corrupt insider working for the criminals.

Season 1 of Line of Duty covers episodes 1-5. Here's everything that happened:

Season 1, Episode 1: A Disastrous Affair - DS Steve Arnott is transferred to AC-12 after refusing to cover up a botched counter-terrorism raid that killed an innocent man. His first case targets DCI Tony Gates, a celebrated officer suspected of manipulating crime figures. Gates is having an affair with Jackie Laverty, who commits a hit-and-run. Gates helps her cover it up, ultimately deleting crucial evidence linking her to the victim, further entangling himself in corruption.

Season 1, Episode 2: The Assault - DCI Tony Gates attempts to sever ties with Jackie Laverty, but DS Arnott and DC Fleming's investigation uncovers her money laundering. Gates realizes Jackie deliberately killed her accountant and arrests her. After Jackie manipulates him, they return to her home. The episode concludes with masked men breaking in, slitting Jackie's throat and planting the murder weapon in Gates's hand, leaving him framed for the crime.

Season 1, Episode 3: In the Trap - Steve Arnott investigates Tony Gates at Jackie Laverty's crime scene, suspecting a cover-up. AC-12's interview with Gates proves challenging as he skillfully evades their questions. Gates is kidnapped and shown Jackie's body, then blackmailed into working for criminals. Frustrated by his inability to expose Gates and feeling compromised, Steve Arnott quits AC-12, believing Gates has won.

Season 1, Episode 4: Terror - Blackmailed DCI Tony Gates attempts to reframe the murder investigation as a terrorist plot. DS Steve Arnott returns to AC-12, and with Kate Fleming, uncovers Gates's financial ties to Jackie Laverty. Gates is forced to lure Arnott, who is then abducted and brutally tortured by Hunter's gang. The episode concludes with Kate discovering Arnott's last known location, racing to save him.

Season 1, Episode 5: The Probation - DCI Tony Gates rescues DS Steve Arnott, then works with AC-12 to apprehend Tommy Hunter. Realizing his career is over, Gates commits suicide by walking into traffic to secure his family's pension. Arnott and Fleming report his death as "in the line of duty." The episode concludes with the shocking reveal that DC Matthew "Dot" Cottan is Tommy Hunter's corrupt inside man, "The Caddy".