In the fourth and final episode of "The Breakthrough," titled "A Lonely Person," the investigation into the 2004 double murder in Linköping reaches its climactic conclusion. The episode brings together the threads of the story, culminating in the identification and arrest of the killer, David Nilsson, a reclusive and mentally unstable individual whose actions had haunted the city for over a decade.
The episode begins with Per Skogkvist, the genealogist, needing more DNA samples to confirm the killer's bloodline. Desperate to make progress, the police squad and their acquaintances agree to get swabbed. Meanwhile, reporter Stina Eriksson, eager to get ahead of a snooping colleague, secures an exclusive interview with Per, who agrees on the condition that she also provides a DNA sample.
As the investigation intensifies, Per narrows down the region to Algbosatter but pauses his search when he learns about the arrest of Ante, a suspect identified by the coach who had felt guilty for not speaking up earlier. Ante's refusal to get swabbed back in 2004 and his proximity to the crime scene make him a prime suspect, but his DNA does not match the killer's profile.
The breakthrough comes when Per, working from home to care for his daughter Klara, discovers a strong match with Stina's twin cousins, David and Stefan Nilsson. After rechecking his family tree multiple times to ensure its accuracy, Per confirms that David is the strongest match. John Sundin, confident in Per's findings, requests an arrest warrant, and a tactical unit is assembled to apprehend David.
The arrest of David is smooth, and his DNA matches the profile from the crime scene. During the interrogation, David confesses to the murders, explaining that he was driven by a voice in his head telling him to kill two people to find peace. His confession brings a sense of relief and closure to the investigation, which had been a source of frustration and obsession for John and his team.
The reactions to the arrest vary among the characters. Stina, initially shocked, learns about the tactical unit and threatens to expose the case online, prompting John to tell her she is the missing piece. Gunilla's husband, Kjell, is informed of the arrest and reveals that he had dreamt of her that very morning. Adnan's family, including his father Saad, express their gratitude to John for keeping his promise and solving the case.
In the aftermath, John decides to reconnect with his son, Henry, apologizing for his absence and expressing his desire to be part of his life. Henry, moved by his father's success in the case, agrees to let John back into his life. Per, meanwhile, finds renewed purpose in his lectures, which become popular, and eventually collaborates with Stina on a book about genealogy.
"A Lonely Person" is a poignant exploration of the human cost of a long-standing murder investigation. It highlights the dedication and perseverance of those involved in the pursuit of justice, as well as the emotional toll it takes on both the investigators and the victims' families. The episode serves as a fitting conclusion to the series, emphasizing the importance of innovative methods in solving complex crimes and the power of human connection in overcoming adversity.