The shooting that left DCI Morck badly wounded, paralyzed his partner Hardy, and killed another officer remains unsolved during the series. Morck suspects it was an inside job linked to DC Anderson, who had ties to a local crime boss and had suspiciously called in a fake wellness check before the shooting, but the assailants are not definitively identified in the 2025 series.

Merritt Lingard, a former controversial prosecutor believed to have drowned four years prior, is the central cold case that Morck and his team investigate. Akram Salim suggests looking into Lingard's disappearance, which becomes the main focus of Department Q's efforts, leading to breakthroughs that energize the team.

Carl Morck and Akram Salim develop a strong partnership marked by complementary skills and personalities. Akram, a Syrian former policeman, insinuates himself into Morck's basement office and becomes his de facto partner. Their dynamic evolves like a classic buddy cop relationship, providing both plot advancement and moments of humor.

Rose Dickson is a young constable who joins Morck and Akram in Department Q. She contributes to the investigation of cold cases, including the Lingard disappearance, and is part of the team chemistry that balances the group and helps advance the plot.

Morck's trauma from the shooting, which left him wounded and his partner paralyzed, causes him guilt and isolation. He is regarded as arrogant and antisocial by colleagues and is required to attend therapy sessions. This personal struggle influences his difficult relationships at work and his initial exile to the basement office, shaping his character's vulnerability and determination in solving cold cases.