Is the information they bring worth the cost of protecting them for the rest of their lives? Is it even genuine? Might they be double agents? These are some of the questions facing MI6 when a Russian agent hands himself in to them in Denmark. As a team begins to assess his value, his former employers in the Kremlin develop a brutal plan to show that no defector will ever be safe. And they know where to find him. Which means there must be a mole in MI6. So it is that the cavaliers of Six find themselves being interrogated by nondescript Jonas Merrick of Five - the man called back from retirement and his beloved caravan, the man the young guns call the Eternal Flame because 'he never goes out'. But while he may be grey, Jonas is also ruthless. As he quietly works through the suspects in London, and violent mayhem breaks out in Denmark, Jonas plans not just to unmask a traitor, but to hit back at the Russians with deadly force.