The explosion is powerful and the devastation enormous when a subway train explodes in Stockholm's tunnel system on a November night. The country is in shock. A group calling themselves "The Holy Riders of Siffin" claim responsibility for the attack in an anonymous call that the A Unit manages to trace. They think they are close to an arrest. What they don't know is that this is just the beginning of a hunt framed by political extremism, hate and death. (Part 2)

Investigating a series of murders and kidnaps, 'A' Unit begin to suspect they are linked to a shared high school experience.