The surveillance operation begins at 4:00 pm. Otto, Waaler, Sivert Falkeid from Special Forces, and Harry monitor screens in the bus, while Falkeid’s teams are positioned throughout the building. Nothing happens until well after 5:00 pm; there is a false alarm, but no one enters the building or tries to harm any of the students.

Meanwhile, at Olaug Sivertsen’s home in Villa Valle, Beate Lønn keeps watch while having tea with the elderly woman. Olaug explains that her son Sven is the child of a German soldier, had a difficult upbringing, and lives in Prague, although he has been visiting her frequently of late. He is expected in Oslo that evening. Olaug shows Beate a brooch Sven recently gave her, featuring a diamond cut into a pentagram shape. Beate immediately recognizes its significance, requests a recent photograph of Sven, and calls Harry to identify Sven Sivertsen as the suspect. Waaler orders backup headed to Olaug’s house to stand down but keeps Falkeid’s team at the student building. He leaves Harry in charge of surveillance and departs alone to handle Sivertsen’s arrest.

Alongside these present-day events, a series of flashbacks occur. Marius Veland, a young media studies student who lives in the student residence, gazes out his window and sees a bike courier outside. Moments later, the courier knocks at his door and then enters the room and holds him at gunpoint. The courier forces Marius to write a letter to his parents stating that he is going abroad for several weeks and leave a similar note on his door. After Marius completes these tasks, the courier shoots and kills him. These events took place before the day of the stakeout.

Tom Waaler arrives at Olaug Sivertsen’s house, confirming that no other officers are coming and that he and Beate will handle the arrest alone. Meanwhile, while watching the student residence via the surveillance cameras, Harry notices a pentagram carved into the plaster above room 406 (Marius’s room). He rushes to the room, where a note on the door claims the occupant has gone abroad. Inside the room, he finds a preserved human thumb on a shelf.

Harry contacts Marius’s parents and learns from the postmark on their letter that Marius Veland disappeared approximately twenty days earlier: five days before Camilla Loen’s murder. Harry realizes that Marius was the killer’s first victim; the student residence is the site of the first crime, not the fourth. He calls Forensics to process the scene, then phones Beate. He warns her that based on this new information, the killer will strike at Villa Valle that night (which matches up to Olaug sharing Sven’s plan to come to her that night). Harry questions why Waaler is conducting the arrest alone, noting it violates standard procedure.

Bjørn Holm from Forensics arrives at room 406. Harry’s attention is drawn to melted rubber on the floor, which could be used to create an airtight seal. Noticing the preserved thumb pointing upward, Harry deduces it is a clue indicating the floor above: the loft. Harry, Falkeid, and two officers proceed to the loft. Falkeid is skeptical since a police dog swept the loft the previous day and would have detected any decomposition. Harry suggests the body was sealed to prevent odor from escaping.

Using keys from Marius Veland’s room, Harry unlocks the storage space assigned to room 406. A heavy suit bag falls from inside a wardrobe, knocking Harry to the floor. Through the sealed plastic, he sees the face of Marius Veland’s corpse, hidden in the loft for weeks.

At 9:56 pm, Beate and Olaug hear footsteps approaching the house. Olaug identifies them as her son Sven’s.

In the hallway, Waaler repeatedly shouts commands for Sven Sivertsen to drop his weapon. Growing suspicious, Beate enters the hall with her revolver drawn and observes that Sivertsen is carrying only a suitcase. Waaler has already positioned himself to fire. Beate informs Waaler that Sven has no gun, and after a tense pause, Waaler orders her to handcuff the suspect. The arrest proceeds without violence.

Near midnight, Bjarne Møller conducts a press conference confirming Sven Sivertsen’s arrest, telling reporters that weapons and diamonds matching those found on the victims were discovered in his possession. Harry and Beate watch from a distance as Waaler basks in media acclaim. Beate notes that Waaler is being portrayed as a hero despite the uneventful arrest. Harry tells Beate this was his final case and that he is leaving the force.

As Harry prepares to drive home, Waaler catches up to him. Waaler reveals he leads a vigilante organization that eliminates criminals the justice system fails to punish and assigns Harry his first task: to murder Sivertsen in his cell using an undetectable KGB-developed poison that causes respiratory failure. Waaler explains that the death will be passed off as a suicide and that it’s necessary to kill Sivertsen lest he get away with his crimes. Harry shares his theory that Waaler was going to kill Sivertsen during the arrest, and this is why he kept Beate away until she interrupted. Waaler continues to insist that the justice system is no longer adequate.

Waaler gives detailed instructions to the cell where Sivertsen is being held until Sunday night, and where Harry can find the key to access it. Waaler gives Harry 24 hours to complete the job.