Maria Kallio is a police officer who is one final exam away from earning her law degree. She has grown weary of her studies, however, and so she accepts a temporary position as the leader of the Länsi-Uusimaa Police's murder squad. Her first case is complicated by the fact that Maria is acquainted with both the victim and the main suspect, Antti Sarkela, from her university days.

Maria's boss wants to move her from temporary to permanent status, replacing the current squad leader. While investigating the murder of a strangled young doctor, Maria realizes she likes being in charge of the team. As Antti expresses his desire to move their romantic relationship forward, Maria also notices how well she gets along with her colleague Koivu.

Antti's surprising decision to do research in America convinces Maria to set aside law school and accept the permanent position as head of the murder squad. She moves to a cottage in her old hometown and runs into her teenage crush, Johnny. But when the chief geologist of a mining operation is murdered at the company's opening event, Maria begins to suspect Johnny's involvement.

A body wrapped in plastic is found in the sea, and crime scene investigator Pulkka identifies him as the preacher of a local spiritual community. To solve the murder, Maria needs the help of Koivu's new girlfriend, Anni, who works in the financial crimes unit. Although Antti has returned to Finland, Maria finds herself fighting feelings of jealousy about Koivu.

Maria faces intense pressure and prejudice as she investigates the murder of a young girl from an immigrant family. Ström, the former leader of the murder squad, is now working for the Security Police and demands that Maria provide information about the victim's family. On the home front, Maria's father has come to visit, and Antti suggests they find a new apartment and move in together.

After a prostitute dies by poisoning on a live TV talk show, Maria discovers that a Russian-backed group has taken control of sex trafficking in the Helsinki area. Maria orders Susanna and Koivu to infiltrate a restaurant that is at the center of the trafficking, but the undercover operation soon turns deadly.