In London 26-year-old Venetia Stanley - aristocratic clever bored reckless - is having a love affair with the Prime Minister H. H. Asquith a man more than twice her age. He writes to her obsessively sharing the most sensitive matters of state. As Asquith reluctantly leads the country into war with Germany a young intelligence officer is assigned to investigate a leak of top secret documents - and suddenly what was a sexual intrigue becomes a matter of national security that will alter the course of political history.