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 1)

A woman is found collapsed in the street, having escaped from an unknown basement. She has no idea why she was kidnapped and, with nothing else to go on, 'A' Unit follow a lead found in the basement to a drugs ring run by two brothers.