When a steel industry executive is found dead in a burning BMW, the death is quickly dismissed as a tragic accident. After another nearly identical accident occurs, criminal inspector Eva Nyman receives a letter that reveals details of an upcoming, and much more vicious, attack, leading her to believe that these are a deliberate series of murders, possibly even a case of climate terrorism.

In the letter Eva recognizes a phrase that was often used by her disgraced former boss, Lukas Frisell, who went off the grid after botching a kidnapping investigation. Eva cannot believe that Frisell is responsible for the murders, but evidence keeps pointing toward him. And as her new team, NOVA, races to uncover the murderer before more people die, Eva is reluctant to disclose her suspicions regarding her old boss.

As further bombings targeting public places begin, countless lives rely on NOVA's ability to find out who's behind it all and put a stop to the terror. The killer is out there somewhere, but all they have to go on is a cryptic letter and a strange symbol that keeps appearing everywhere they turn: a circle within a circle.

Who is hiding within the circle?