Klaus Mollert is at home playing video games while his babysitter and her boyfriend make out on the couch. After drinking an energy drink, Klaus takes out a gun and shoots at the game screen. The babysitter hears the shot but her boyfriend dismisses it. As they go back to kissing, Klaus enters the room and shoots them dead, and then kills the parakeet.
Later, Gunter and Karl are called to the crime scene after Klaus's single mother comes home from a date. Karl wonders why Gunter personally requested the case and the hitman says that it restores his faith in young people. They talk to a shaken Mrs. Mollert, who insists that Klaus is a good boy. When Gunter expresses his admiration for Klaus, Karl draws him to the side and tells him that Mrs. Mollert is grieving and to avoid contradicting anything she says. When she blames herself, Gunter is glad to agree with her. The detectives check his room and finds lots of images of violence. When Gunter says that Klaus is part of the necessary evil of life, Karl wonders what made him that way. Gunter says that he was born on the dark side. They search the room and Gunter notices dozens of cans of energy drinks, while Karl finds an e-mail from one of Klaus' friends, Fassbinder, with a photo of a gun.
Tannhauser is playing chess against a champion and loses. When the player admits that he feels guilty for being paid to beat him, Tannhauser says that his opponent has won many battles but he wins wars. When the chess champion points out it hasn't helped Tannhauser beat him, Tannhauser says he might beat him next time. Martine comes in to tell Tannhauser that his new partner called to say that the drug shipment has arrived safely in Tangier using Tannhauser's new delivery method. The mobster says that he's accomplished checkmate against Racken. When the player asks about Martine's pregnancy and if they know if it will be a boy or girl, Tannhauser asks the same question but then says that it doesn't matter as long as the child possesses his seed.
Gunter, Karl, and Braden go to see Klaus, who is locked up in an interrogation room and having a psychotic episode. The drug test shows there are designer drugs in his system. Gunter offers to go in and talk to the boy, taking a blow dryer with him. He offers Klaus popcorn in return for information on Fassbinder and where Klaus bought the gun. When Klaus attacks him, Gunter easily subdues him and pours unpopped popcorn kernels down his throat and then starts popping them with the hair dryer. Klaus finally tells him that Fassbinder is a friend at school and gave him the gun and the drugs.
At the school, Gunter and Karl talk to Klaus' teacher, who insists that he's a model student until he started failing after his father left. Two students fight and a girl runs to the window to vomit, and Gunter points out that the teacher has no control over her students. When he does, the teacher starts flirting with him. Karl asks about Fassbinder and she explains that he's a custodian that Klaus sought out as a father figure.
The teacher takes them to Fassbinder, who is in the hall drinking an energy drink. He runs as soon as he sees them, opening fire, and the detectives run after him. The custodian runs into the middle of a basketball court and Gunter shoots down the players. Karl shoots the last one when he attacks them and they tell Fassbinder to drop the gun. He drops from a heart attack before they can shoot him.
When the detectives get back to the station, Braden tells them that Fassbinder died of cardiac arrest. He was high on a mix of drugs and Karl wonders how they got them. Gunter shows them a can of the energy drink and figures that since Klaus and Fassbinder both had them and that the school vending machine does as well, the drink is spiked.
Gunter and Karl go to Glücklichkeit Beverages, the manufacturing company, and talk to the owner, Rolf Dortmunder. He claims that someone is tampering with their program but insists that they have no disgruntled employees. Karl says that they'll be back the next day with a warrant. As they leave, Dortmunder's secretary, Mrs. Bartleby, comes in. Dortmunder tells her that some of the cans with drugs found their way to the school. Bartleby says that the driver must have screwed up and wonders what to do. He figures that they have to do what they'd do if they had drugs at home.
That night, Gunter goes to see Klaus at the hospital. The boy has learned that Fassbinder is dead and Gunter tells him that the people who tampered with the drinks are to blame. Gunter assures him that he won't be accountable and will send a few years in a mental home. When Klaus says he doesn't believe him, Gunter agrees and says that adult lie to children all the time. He tells the boy that he has a child coming and hopes that it's just like Klaus. When Klaus complains that his restraints hurt, Gunter releases him and goes.
Tannhauser is contemplating his chessboard when Martine tells him that the drugs were delivered to local vendors. Racken controls the trucking and Tannhauser figures that his rival is responsible for the misdirected drugs. Martine tells Tannhauser that Dortmunder is dumping the rest of the shipment but Tannhauser is more interested in seeing the ultrasound. She shows him and he congratulates her on a job well done.
Gunter wakes up in bed next to the teacher after another nightmare about Martine. As they have sex, Martine calls to tell him that Tannhauser is suspicious that the baby isn't his. When he tries to hold the teacher off, Martine thinks she's talking to him and hangs up. Karl calls to tell him to get down to the station right away.
At Glücklichkeit, Dortmunder's people and their children are pouring the drugs down the toilets. Bartleby congratulates Dortmunder on his quick thinking and he says that Tannhauser thanked him by sending a box of cigars. As Bartleby goes to get some matches, Dortmunder opens the box and sets off the explosives.
At the station, Karl tells Gunter that people are rioting after the explosion. Braden tries her coffee and complains about the test, and Gunter realizes that the drugs are in the water system. Karl figures that Gunter is unhappy because everything is caused by chemicals, not evil. Klaus calls Gunter and tells him that he's gone back to school to be what he was meant to be: bad. He then hangs up and leads the teacher away at gunpoint.
Karl and Gunter go to the school and are informed that Klaus has taken a hostage. Gunter goes in alone and Klaus says that he's going to kill the teacher and get all the credit. The hitman says that if he kills again it will fall under the third strike law and that Klaus can still walk away because it was the drugs. When Klaus wants to kill the teacher for giving him bad grades, Gunter says that he was wrong about Klaus and it's not who he is. He also tells Klaus that no woman is worth killing and to give the gun to the teacher. He does and the teacher shoots Gunter in the face just as Karl runs in and tells her to drop it. Mrs. Mollert runs to her son while Gunter laughs that he likes being shot in the face.
Later, Tannhauser loses another game of chess and then beats the chess champion to death while Martine has a drink and watches.