The landline rings as Agneta is waving off her grandchildren. Just one word comes out of the receiver: 'Geiger'.

For decades, Agneta has always known that this moment would come, but she is shaken. She knows what it means.

Retrieving her weapon from its hiding place, she attaches the silencer and creeps up behind her husband before pressing the barrel to his temple.

Then she squeezes the trigger and disappears - leaving behind her wallet and keys.

The extraordinary murder is not Sara Nowak's case. But she was once close to those affected and, defying regulations, she joins the investigation. What Sara doesn't know is that the mysterious codeword is just the first piece in the puzzle of an intricate and devastating plot fifty years in the making . . .