The Players have tired of Earth, except for the Countess. However, she has been outvoted. Play will proceed to an Endgame, and continue until one side can claim total victory, even if this means the destruction of the Earth...

London, 1951. The Doctor has been on Earth for five decades and doesn’t appear to have aged a day. He has given up all hope of ever knowing who he was, and spends his days at the British Library and his nights sitting in the darkness of his rented flat next to an empty blue box. When his Polish acquaintance Oskar Dolinski comes to him, claiming to have acquired documents which his enemies will kill to retrieve, the Doctor doesn’t believe him; Oskar is obsessed with restoring his homeland’s rightful government and sees conspiracies around every corner. But that night, on his way home, the Doctor sees a group of thugs trying to drag Oskar into a rented car, and instinctively intervenes, driving them off. He offers to let Oskar sleep in his flat until he’s sure it’s safe, but Oskar feels guilty about involving the Doctor and slips out while the Doctor is sleeping, leaving the stolen documents behind. Outside, however, he is picked up by the thugs and taken to the Russian Trade Delegation headquarters, where his kidnappers beat him to death but fail to learn where he left the documents.

British agent Guy Burgess has been sent home from Washington in disgrace due to his debauchery, drunkenness and open homosexuality, and his fellow agent Kim Philby has returned as well while tempers cool. Or so he claims. Young Jimmy Melville is on the trail of the stolen documents, and Philby intends to keep an eye on his progress. Both Philby and Russian cultural attache Vasili Mikoyan are well aware that if the documents reach their superiors, or even ambitious underlings, the consequences could be disastrous. Krychov, the officer in charge of Oskar’s interrogation, has reported his failure to Mikoyan, who concludes that Oskar must have been a field agent and that the mysterious Doctor who seemed so well trained in the martial arts was his superior. He thus orders that the Doctor be brought in for questioning.

The next day, the Doctor reads of Oskar’s death in the newspapers, and, stunned, decides to mail the documents to himself to keep them safe. He is intercepted on his way out of the post office by Krychov and his men, however, and taken for interrogation. Fortunately, Melville has been keeping the Doctor under surveillance, and he rescues the Doctor before the interrogation can get seriously brutal. However, before anyone can stop him, the Doctor lashes out at the thugs who killed Oskar, crippling them both with a single blow. He is sedated and sent to hospital, where he recovers and escapes before the British agents can begin to question him. Philby has arranged for the Doctor’s package to be intercepted by the postal authorities, but the Doctor uses stolen ID to reclaim it. By this time, however, Philby has been reminded of a report he received from Graham Greene during World War Two, of a bizarre incident which also involved a mysterious, nameless Doctor. Melville searches the Doctor’s flat and finds only a large blue box, and Philby gives him certain instructions...

Hoping to trade the documents for his solitude, the Doctor tries to decode the message. All he can determine, however, is that it appears to be a communique across Cold War lines, suggesting that there is a traitor on at least one side. As night falls, he returns to his flat, only to find his blue box missing and a note with an address left in its place. He is then attacked by a blond assassin named Axel, who seems to know him of old, and who claims that he’s just trying to heat up the Cold War as part of the game. The Doctor overpowers Axel, but as he forces Axel to point his gun at his own head, Axel seems to vanish into thin air.

The Doctor proceeds to the address in the note, where Philby is waiting for him. The message was meant for Philby all along, but it had been intercepted by a well-meaning Polish spy who thought they contained Soviet secrets. This is the real reason Philby returned from Washington; he is in fact a double agent for the USSR, and has used his unique position to set up a world-wide network of agents on both sides of the Iron Curtain. The organisation, known as Tightrope, is intended to monitor leaders on both sides, and ensure that no madman or fool tries to start a nuclear war in the mistaken belief that it could be won. Lately, however, it seems that a group called the Players is working against Tightrope’s objectives and intensifying the Cold War, apparently trying to provoke a nuclear exchange which could destroy the planet...

The Doctor finds himself oddly unsurprised by the concept of people who would try to destroy their own world; perhaps it isn’t theirs at all. While remaining sceptical about the Doctor’s more far-fetched theories, Philby nevertheless believes that the Doctor has some connection to these Players, even if he doesn’t know what it is himself. He therefore refuses to return the Doctor’s mysterious blue box until the Doctor agrees to help him smoke out the Players and stop them. The Doctor’s first task will be to smuggle Philby’s fellow double agent Donald Maclean out of the country; the message which Philby has just decoded is a warning that Maclean is about to be arrested, and if he cracks under interrogation then Philby -- and Burgess -- will also be compromised. The Doctor therefore travels to Beaconsfield, Maclean’s country home, to tell him that he must celebrate his birthday by abandoning his wife and children and fleeing his country to live as an exile in Moscow. As he tries to convince Maclean of the urgency of the situation, Burgess arrives to deliver the same warning, and reveals that he’s seen someone watching the house. Believing that Maclean has been placed under surveillance, Burgess decides that he must defect as well or else face having to explain why he visited Maclean’s house the very night Maclean fled the country. The Doctor is more concerned when Burgess’ describes the man he saw outside; it appears that the man watching the house is Axel...

When Burgess and Maclean go out to Burgess’ car, Axel is waiting there to kill them, but the Doctor surprises and overpowers him. Axel soon recovers, pursues them as they flee and tries to drive them off the road, but Maclean shoots out the pursuing car’s tyres, and they continue on to the ferry without incident. As Burgess and Maclean drown their sorrows, however, Axel impossibly appears aboard the ferry, knocks them both out and takes the Doctor prisoner. He intends to kill both Burgess and Maclean and leave their bodies in a compromising position, embarrassing British intelligence and causing further international tension; however, this is merely a side game, for bonus points. The real Endgame is taking place elsewhere. The Doctor once again overpowers Axel and throws him overboard, and the rest of the crossing is completed without incident. As Burgess and Maclean set off for Moscow the Doctor flies to Washington to report success to Philby, and on the way, he ponders the allegiances of the men he has helped to defect. They have given up their country and families for what they believe in; does that make them heroes, traitors, or both? And what does the Doctor believe in?

Philby is disturbed to learn that Burgess has defected as well, and worries that he may fall under suspicion himself; but first he must deal with the Players, and thus he gives the Doctor all of his files on the Player phenomenon. The Doctor reads them all and still has time to help Philby’s son John repair and play with his model trains before Philby returns. It appears that a number of diplomats and soldiers, both Russian and American, have suddenly started to attack people on the other side for no apparent reason. Most of the incidents have been hushed up, but the Doctor fears that the troubles could escalate if a cause is not found. All those affected had been in Washington at some point before they snapped; what if there is some force at work in the Capitol area, pushing people’s natural Cold War paranoia into something else... and what if it starts to affect President Truman? When Philby remains sceptical, the Doctor loses his temper and demands that Philby just leave him alone. But Philby still needs his help, and even the Doctor must concede that if Earth is destroyed, he will go along with it.

That night, Philby and the Doctor dine with Jim Anderson from the American Secret Service, who tells them that the President’s aide Kent Howard has recently voiced concerns about Truman’s behaviour. The Doctor visits the White House to speak with Howard, who claims that Truman has become more militant and aggressive and has recently advocated dropping atomic weapons on Peking whereas he had previously favoured a peaceful end to the war in Korea. The Doctor finds that Truman’s bouts of aggression coincide with visits from Professor Myrek, the head of a secret CIA project to test the weapons potential of psychic powers. When Truman arrives to speak with Howard, the Doctor convinces him that he’s interested in starting a psywar project in Britain, and Truman thus agrees to let him visit Project Kali. Myrek and his assistant Helga Stoll let the Doctor see their most promising cases, hoping that he will take a good report back to Truman and thus allow them to get closer to the President and boost his conditioning. The Doctor, however, realises that Myrek is using his own psychic powers to enhance his results, and accuses him of faking his results to ensure continued funding. He demands more conclusive evidence of their success, and Myrek, believing that the Doctor is unaware of his attempts to influence Truman, shows the Doctor Kali’s mind-control laboratory. Here, subjects have been connected to telepathic amplifiers and mentally linked to another mind at a distance. As the Doctor watches, Myrek orders a subject to lash out at those who would belittle him; however, the target mind resists the violent impulses, and the psychic trying to control him dies of a brain haemorrhage. Promising to tell the President what he has seen, the enraged Doctor goes straight to Howard. Truman has left the country for a few days, but when he returns Kali must be shut down -- and Myrek must be kept away from Truman at all costs.

After avoiding another attempt on his life by Axel, the Doctor returns to London with Philby, who has been recalled to face an inquiry into the defection. On the way, Philby orders the Doctor to defect to Moscow and find out if Stalin is also being influenced by the Players. He admits that he has another reason for asking the Doctor to go; the British are aware that a third man helped Burgess and Maclean to defect, and by telling them it was the Doctor, Philby will divert suspicion from himself. The Doctor has had enough and refuses, demanding his blue box back, but upon their return Philby frames the Doctor for espionage and has him arrested. The Doctor escapes easily, but has no choice now but to seek sanctuary at the Russian embassy, where Mikoyan arranges for him to be sent to Moscow.

In Moscow, the Doctor is reunited with Burgess and meets Igor Timenko, a Tightrope agent who tells him that the dying Stalin has recently come under the influence of a mysterious psychic healer. Timenko smuggles the Doctor into Stalin’s dacha, where the Doctor sees a strangely familiar woman whispering to the sleeping Stalin, fueling his paranoia and driving him further into madness. Furious, the Doctor confronts the woman he once knew as the Countess, and she is delighted to see her old opponent at first -- but soon realises that there is something terribly wrong with him. The Doctor still champions humanity and urges her to leave them to their own devices, rather than playing cruel games with their destiny; but his heart is no longer in his arguments, he doesn’t seem to recall their past encounters, and the sheer joy of living he once exuded is nowhere to be found. The Countess offers to restore the Doctor’s lost memories, but to her shock he collapses in terror as she approaches, shrieking that he mustn’t know. Upset, the Countess sees the bewildered Timenko watching them, and has him take the Doctor to safety. For the Doctor’s sake, she undoes the damage she has already caused, telling the sleeping Stalin that he is victorious and that his enemies are too frightened of him to attack.

When the Doctor recovers, Timenko puts him aboard a delegation to Washington, where the Doctor contacts Howard to find out about Project Kali. Howard has given his report to Truman, but when he tries to contact the President he learns that the furious Truman has set off for Project Kali to see the truth for himself. The Doctor, Howard and Anderson rush to the rescue, and while Howard and Anderson deal with the Marine guards on the project grounds, the Doctor slips past them to the mind-control laboratory. Axel, Myrek and Helga are reprocessing Truman, who is struggling to resist them but can’t hold out for much longer. Axel, claiming victory, prepares to shoot the Doctor -- but then the Countess steps out of the shadows, and telepathically forces the other Players to kill each other before they realise what’s happening. The Countess then departs, telling the Doctor that she always believed there were more games to be played on Earth, and promising that they will meet again. When Howard arrives, the Doctor tells him the Players killed themselves when they realised their conspiracy had been exposed.