The Americans launch a bombing raid on Rjukan against Tronstad's objections, but the only result is heavy civilian casualties. The Germans decide to move the entire heavy water facility to Germany, and the Allies plan a strike at the weakest link along the route.

The Americans insist on bombing the heavy water factory, but Tronstad persuades the Allies to send in a team of Norwegians instead. In Germany, Nobel Prize winner Werner Heisenberg promises a breakthrough in the development of a Nazi atomic bomb.

The Gunnerside team is ordered to sink the ferry carrying the heavy water facilities midway across the lake from Rjukan. Werner Heisenberg, knowing the heavy water is on its way, needs only 500 additional liters to succeed with the German atomic bomb.

In Berlin, Werner Heisenberg is appointed director of the Kaiser Wilhelm Institute. The Gunnerside team succeeds in blowing up the Hydro factory, but the Germans mount a major search for the saboteurs and order an immediate rebuilding of the heavy water facilities.

More than a decade after quitting the force to start a family, former detective sergeant Marcella Backland is devastated by the breakdown of her marriage. When a detective comes seeking her opinion on an unsolved case she had investigated, Marcella returns to police work to try to catch the killer.

Tronstad establishes contact with the War Ministry and a plan to destroy the Hydro facilities is drawn up. In Rjukan, new managing director Erik Henriksen is confronted with sabotage of the heavy water facility.

Marcella and the team search for the killer's most recent target-a six-year-old girl-desperate to find her alive. DNA evidence leads to an arrest, and DI Williamson investigates Andrew Barnes's death.

Police locate the blue truck near where the last victim was found, which leads them to the place where he was held captive. As Marcella's team examine the killer's lair, they are horrified to find evidence of more victims than they originally thought.

As Marcella puts the final pieces together, she discovers that Edward is being held captive and races to save him. Later, Marcella decides she's finally ready to face her demons, but will she be able to handle the truth?

As Marcella undergoes therapy for her blackouts, the police find DNA linking Leo's death to convicted pedophile Phil Dawkins, but they need more evidence to hold him. Marcella also questions Reg, a former rock-band member with health issues.