The head of the US National Security Agency is assassinated in a spectacular set piece killing at Heathrow ... An airport employee and his family are found murdered in their council house in Uxbridge ... In New York, a fashionable Upper East Side osteopath receives two postcards showing the Empire State Building ... A group of migrant workers are brutally gunned down in Macedonia ... The quest to find the link between these apparently random events is pursued by Robert Harland - drawn back to a world he thought he'd left behind with a dual role for the UN and MI6.