Having faced imprisonment, torture and worse on the mud planet Rigel IV, Peri asks the Doctor to take her somewhere elegant. He attempts to take her to London in 1899, but instead, the TARDIS materializes in South Africa during the Boer War. A British reconnaissance train has been ambushed and derailed, and as the Doctor and Peri try to sit out the fighting they see an assassin taking a bead on a civilian amongst the British. The Doctor drives off the assassin and tries to warn the civilian of the danger, but the intended victim -- a war correspondent named Winston Churchill -- doesn't believe he's important enough to select as a specific target. With the Doctor's help, Churchill gets the line cleared so the wounded can be returned to base, but is then captured -- as are the Doctor and Peri before they can return to the TARDIS.

The prisoners are taken to a camp in Pretoria, where the Doctor, Peri and Churchill are accused of participating in acts of war and are held along with the military prisoners. The Doctor and Churchill begin to plan their escape, but much to Churchill's surprise, somebody slips him a map of the prison and all the tools he needs. He concludes that he is being helped by an underground escape network, but the Doctor suspects that he is in fact being led into a trap. The Commandant is contacted by a mysterious Captain Reitz, who orders him to ensure that Churchill is shot while attempting to escape. The Commandant refuses to carry out such dishonourable and illegal orders, and the Doctor and Peri catch a glimpse of the frustrated Reitz leaving the Commandant's office -- and recognize him as the man who tried to assassinate Churchill earlier...

Peri flirts with an impressionable young guard and learns that the TARDIS is being kept in a nearby hut normally used to store explosives; since nobody has been able to enter it, it is to be shipped to a scientific base for further study. That night, the Doctor, Peri and Churchill slip out of the camp via the safe route marked on Churchill's map, and Churchill is met by a mysterious woman who guides him to the railyards. There, he hides himself aboard a cargo train, and rides it to safety and a hero's welcome; thus begins an illustrious career in the armed forces... Meanwhile, the Doctor and Peri bluff their way into the hut where the TARDIS is being kept, but Reitz happens along and concludes from the guard's story that the bossy man who entered the hut must be Churchill. He orders the guards to open fire on the hut, which explodes violently, leaving no trace of the Doctor, Peri or the TARDIS. Reitz then vanishes, and the Commandant, rather than admit to having killed two civilians, erases them from the camp records and releases a cover story claiming that Churchill blew up the hut to cover his escape.

Safe in the TARDIS, the Doctor fills Peri in on the background of the Boer War, and admits that he's met Churchill before under similar circumstances. He uses a device which translates thoughts into pictures to show her what happened after his second incarnation was placed on trial by the Time Lords. Although he had been sentenced to forced regeneration and exile on Earth, the Time Lords first demanded that he act as their unofficial agent under certain circumstances in which they didn't wish to appear involved. Before agreeing to do so, the Doctor demanded proof that the business of the war games had indeed been sorted out, and the Time Lords sent him to the battlefields of World War One to see for himself. There, he was reunited with Lieutenant Jeremy Carstairs and Lady Jennifer Buckingham, neither of whom remembered their experiences. Satisfied that all was back to normal, he was about to depart when the ambulance stumbled across a band of deserters who had laid ambush to a British staff car. Carstairs and the Doctor helped to rescue the occupant of the car, Major Winston Churchill, but the driver was killed -- and spoke his last words in German.

Due to the delay, Churchill and his new companions were forced to shelter for the night at a nearby chateau -- where Churchill seemed to be expected. His original driver was supposed to bring him here, into the hands of the Count and Countess, who intended to deliver him to the Kaiser. The Doctor suspected that the two were not really German agents, but were interfering in Churchill's life out of boredom; he also realized that the Countess had taken a shine to Churchill, and advised Churchill to take advantage of that fact. When the plane arrived to take the prisoners to Berlin, the Countess slipped a gun to Churchill at the last minute just as the Doctor had hoped she would. He used it to hold off his guards and get to the plane with Carstairs and Lady Jennifer, and the Doctor held off the Count while the others escaped. The furious Count placed the Doctor before a firing squad, but the Time Lords removed him at the last moment. However, they refused to listen to his warnings that someone was meddling in established history...

Having finished his story, the Sixth Doctor becomes convinced that this incident is related to the experience they've just had, and decides to take Peri to London in the 1930s to investigate further. The British government is still trying to appease Hitler by giving in to his demands, and Churchill has fallen out of favour due to his insistence that Germany is gearing up for war. The German ambassador to Britain, Joachim von Ribbentrop, has been contacted by a mysterious organisation called the Consortium who have a plan which will ensure that Germany and Britain become allies in the coming war. Hitler gives von Ribbentrop all of the resources he needs to ensure that the Plan comes to fruition.

The Doctor accesses his bank account at Cholmondeley's, claiming to be his own descendant, and he and Peri enter high society by spending vast amounts of money as conspicuously at possible. They are soon invited to a garden party at Buckingham Palace, but they have also attracted the wrong kind of attention, and shortly afterwards somebody plants a bomb in Peri's hotel room. They escape this attempt on their life, but the Doctor decides to hire a bodyguard, and Pinkerton's head man in London, Tom Dekker, takes the case. After assuring Dekker that he's no relation to the little man called "Doc" whom Dekker knew in Prohibition Chicago, the Doctor takes Dekker and Peri to the Palace. There, they meet King Edward VIII and his mistress Wallis Simpson; the King's determination to marry her, a twice-divorced American, has caused a scandal in high society. They are also reunited with Churchill, and claim to be descendants of his Boer War acquaintances. At that moment Dekker guns down an assassin who appeared to be aiming for the King, but the Doctor and Peri realize that Churchill was also in the line of fire -- and recognize the assassin as "Reitz", who also hasn't aged a day since 1899...

Churchill invites the Doctor to his country home to discuss the possible conspiracy in more depth, but as the Doctor prepares to go he receives an invitation from Wallis Simpson to meet with her and a special guest. Peri, convinced that the "guest" will be the King, offers to go in his place while the Doctor and Dekker meet with Churchill. Simpson's guests, however, turn out to be von Ribbentrop -- and the Count and Countess who tried to kidnap Churchill during World War One. Peri realizes too late that the invitation was an attempt to get the Doctor into the Count and Countess' hands. At their urging, von Ribbentrop kidnaps Peri, believing that she is an agent of the American Secret Service and an enemy of the Reich.

The Doctor and Dekker are followed from London by a team of SS agents from the German embassy, but the SS are still used to bullying weaker forces and aren't prepared for Dekker to pull off the road and shoot at them with a tommy gun. The Germans flee back to London, and the Doctor and Dekker continue on to Churchill's country home, where his former staff member Colonel Carstairs is surprised that the "Doctor John Smith" Churchill told him about isn't the same one he was expecting. The Doctor concludes from the attempts on his life that he's stumbled across a conspiracy somehow connecting the German military build-up, Wallis Simpson, and the strange attempts on Churchill's life -- but before he can piece together the clues, the Count telephones him and threatens to kill Peri if he keeps interfering. The Doctor and Dekker return to London, where one of Dekker's operatives has tailed Peri to the house being used as a temporary German embassy while the original one is redesigned by Albert Speer. With his help, they raid the embassy and rescue her, and while von Ribbentrop is distracted, Peri knocks him out and takes a list of names which seemed to be important to him.

While at the embassy, Peri overheard von Ribbentrop telling the Count that the signal would be sent from the Fort. The Doctor and Carstairs realize that the stolen list contains the names of Nazi sympathisers in the British government. As they compare notes, Churchill arrives with news that the King, given an ultimatum by PM Stanley Baldwin, has chosen to abdicate in order to marry Wallis Simpson. The Doctor wonders why the King has suddenly caved in after being so obstinate for so long, and Peri wonders how the people of Britain, who seem to love both him and Simpson, will react to this decision. But they realize the truth when Churchill informs them that the King intends to announce his decision to the public in a live BBC broadcast -- transmitted from his summer home, Fort Belvedere.

Churchill convinces both Baldwin and Sir John Reith, head of the BBC, of the danger, and receives their authority to act as he sees fit. Carstairs contacts his friend Colonel Fitzsimmons and has him take his men out to the Fort for a "training exercise". Their preparations made, the Doctor and his friends go to Fort Belvedere, where security is being provided by Nazi sympathiser Sir Oswald Mosley's Blackshirts. The Doctor and his allies stand by as the King announces to the nation that he has chosen to use his regal authority to dissolve Parliament, marry Simpson, and replace the government with one more loyal to its monarch -- with the help of Mosley and his friend Chancellor Hitler of Germany. But before the King can celebrate his victory, Churchill reveals that the Fort is surrounded by Fitzsimmons' men, and that his announcement was not broadcast, but recorded. In order to avoid being placed on trial for treason, King Edward has no choice but to abdicate as he had promised.