Balmoral Castle, Scotland, 1902. The new king, Edward VII, is in residence, relishing a day off. Having waited 59 years to become king he is determined to do his best with the position. Knowing that the following day has engagements in Edinburgh, to be followed by a return to London, he is preparing for a morning relaxing with the newspapers, a large brandy and some fine cigars. Not wishing to let a fine autumn day go to waste, he calls his young companion and aide, Captain Harry Carruthers, and orders him to take a gun and go out on the hills to shoot a stag. Despite his reservations, Carruthers does as he is bidden and sets out to begin his hunt. On his way he is puzzled, when he looks back at the castle. Despite the fine day he notices that it is raining on Balmoral, and the rain appears to be going up.
The TARDIS materializes nearby. The Doctor has found Martha some appropriate clothes: tweed jacket, long skirt, high-necked blouse. They emerge, expecting the Castle to be nearby and are concerned when they cannot see it.
Eventually tracking down his prey, a magnificent twelve-pointer, Carruthers is reticent to pull the trigger. Even with a clear shot he delays. Just as he is about to shoot he is distracted by a shout and fires harmlessly into the air. The shout was from Martha, who berates Carruthers for hunting. Despite being taken aback he is quite happy to agree with Martha’s anti-blood sports philosophy. He tells her he was only obeying his boss’s orders. When Martha sarcastically asks if his boss is the King of England she is surprised by an answer in the affirmative. Carruthers offers to take them to the castle for lunch and an audience with the king, but as they top the next rise they see only a hole in the ground where the castle should be. Both Martha and the Doctor exclaim, “Judoon!”
The readings from the Doctor’s sonic screwdriver show plasma coil traces, indicating that this is, indeed, the Judoons’ handiwork. Carruthers takes surprisingly little convincing that a humanoid race of aliens, with rhino heads, has abducted a whole castle. When the Doctor asks if he has seen any alien devices within the castle, perhaps a small glass sphere with a spinning energy vortex inside it, Carruthers admits that he took just such a thing there in his luggage. The Doctor tells him it is the beacon they will have used to focus their energy beams on. Carruthers says he got the device from his old doctor, Arthur Conan Doyle.
He tells them that Doyle begged him to show the device to the king, hoping to use the king’s authority to get it presented to the top scientists of the Royal Society. Doyle told him that it was a potential energy source of great power but would not reveal where he had got it. The Doctor thinks that Doyle would be a sucker for a friendly alien with an interesting story and sets Martha and Carruthers the task of returning to London to track him down. The Doctor, meanwhile, intends to find where the castle has gone and get inside.
They take Carruthers with them, back to the TARDIS. He is less shocked than they expect at finding a police box (which hasn't been invented yet) with interior dimensions much larger than the exterior. Perhaps his astonishment has been used up by the disappearing castle. A few minutes later Martha and Carruthers are watching the TARDIS disappear prior to hailing a cab back to Carruther’s room so that he can change into his city clothes.
They travel by cab to the Grand Hotel in Northumberland Street and find Doyle in residence. At first he is reticent to say where the device that he gave to Carruthers came from but at Martha’s insistence he eventually tells them. He says that he was invited to join a society called the Cosmic Peacemakers whose work lay on the frontiers of science. They believed it was possible to contact other planes of existence and they gave him the device which, they said, came from another world. They have a laboratory in a sordid part of London on Black Dog Lane near the wharves.
Over dinner at a Soho restaurant Martha convinces Carruthers that they must visit the laboratory. The cab driver who delivers them to the area around Black Dog Lane will not wait, so bad is the reputation of the place. They walk through dark alleys, stepping over the huddled bodies of drunks, and find the steps that lead to their goal. After knocking on the door they see a hatch open and they are told to leave by a skull-like face. When they mention Doyle’s name, however, the door is immediately opened by a white-haired figure in robes that speaks sweetly to them and allows them to enter.
The Doctor puts the TARDIS in a parking orbit around the Earth and traces the plasma coil trail to The Empty Quarter, 250 000 square miles of Arabian desert, the most deserted place on the planet. Landing on top of a nearby dune, the Doctor emerges into the baking heat. Balmoral Castle lies in a shallow bowl before him. Standing beside it are three giant Judoon spaceships. The Doctor can see that a transparent dome is covering the castle, a fact not noticed by a party of horsemen who race down towards it, firing their rifles wildly. They crash into the force field at speed, several are unhorsed, and after a period of confusion, shooting uselessly at the castle, they remount and ride off.
Inside the Cosmic Peacekeepers’ rooms, Martha has the feeling that she has moved to the future again. Glass walls glow with inner light, the furniture is simple and white. The figure who met them at the door introduces himself as Professor Challoner and also brings three other white-robed figures to meet them. Like him they are tall, thin and saintly. Martha does not want to give too much away so she tells them that reports from Scotland indicate that there has been some sort of disaster involving the device. She asks what the device is and where it came from but the Professor says it was discovered in a monastery in Tibet which will take time to contact. At that Martha says there is no point wasting their time further and gets up to leave. She asks to be shown the laboratory but is refused.
Outside in Black Dog Lane she tells Carruthers that the Tibet story is an obvious ruse to play for time. She is intrigued by the futuristic décor she saw and by the fact that the skull-like face that greeted them could only have belonged to Challoner; there wasn’t time for anyone else to arrive or leave, which suggests some sort of shape changer. They decide to go back to the Grand and ask Arthur Conan Doyle some more questions.
There is a surprise for them. Doyle has no memory of their previous meeting, even though it was only a few hours earlier. Martha guesses that their mentioning of Doyle’s name to Challoner has led to the writer having his memory wiped. Martha tries to tell the Doctor all this but her mobile merely emits a high pitched whine.
The Doctor uses his screwdriver to make his way through the thick wall of the force field. Entering the castle he has to hide from six Judoon in their familiar black armour and domed helmets. As he hides someone presses a gun to his head.
Martha returns to Carruther’s rooms where he gives her a set of denim overalls. They return to Black Dog Lane, climb a drainpipe onto the roof of a warehouse and then find a skylight in the roof of the Cosmic Peacemakers’ lair. Dropping down, they make their way through corridors until they arrive in a large, bright chamber. At its centre is a crystal sphere with a fiery vortex at its centre and a bowl of equally fiery liquid beneath it. The console beside it has dials with alien symbols. Martha suggests that it is some sort of power source. Professor Challoner agrees. He is behind them and says he has been expecting their return. He briefly shows them his true, lizard-like, form before telling them that it is his plan to take over the world.
Two Judoon enter the room through a glowing archway. Challoner orders them to execute the two spies but they are stopped by the arrival of their Captain who says that native life forms cannot be destroyed without a trial. All of them leave the chamber through the arch and find themselves in Balmoral Castle.
The man with the gun introduces himself to the Doctor as George Fitzroy. He says that there was a rainstorm where the rain went up, and then Scotland disappeared, leaving only desert, three buildings dropped from the sky and the Judoon marched out.
Martha and Carruthers are led by their captors to a sitting room and introduced to the king. After some small talk about Carruthers’ latest failure to shoot a stag and how pretty Martha is Challoner intervenes, demanding to explain his Master Plan. The Doctor bustles in saying how much he likes Master Plans, though they didn’t work for Genghis Khan or Napoleon and the Daleks seemed to bring out a new one every week. After Carruthers has vouched for the Doctor to the king, Challoner explains that he has a plan that will make the king of Britain into the Emperor of the world.
When the king points out that France, Germany and Russia have armies and navies at their disposal that will have something to say about such plans Challoner explains his methods. He has placed Temporal Reversion devices in all the major capital cities of the world that will eradicate them, their leaders and their people from history, allowing Britain easy victories. Despite Carruthers and Martha objecting to this plan the doctor seems to think that it has some merit and asks for ten minutes alone with the king to convince him to go along with it. After briefly changing back to his lizard shape to show who they are dealing with the professor agrees.
Once the Judoon and Challoner have withdrawn, the king explains that in all his years of waiting to succeed to the throne he had expected to be a king that stopped world wars, not started them. The doctor hastily agrees with him, saying that Temporal Reversion is so unstable that even the Time Lords did not use it. However, to avoid all of them being killed and the same plan being presented to another ruler he wants the king to agree to it. The Doctor gets a quick de-brief from Martha about the Cosmic Peacekeepers and their laboratory near Black Dog Lane. When Challoner returns the king tells him of his agreement to the plan on the conditions that Balmoral is returned to its proper place and the Doctor being allowed to act as the king’s agent. Challoner agrees to the first but is doubtful about letting the Doctor go, even when he explains all the lobbying in Parliament that will have to be done for such a plan to be endorsed. Eventually he gives in to the Doctor but insists on keeping Martha hostage.
The Judoon Captain leads the Doctor to the force field and opens up a pathway. The Doctor says that he cannot believe such honourable people as the Judoon are acting so unlawfully but the Captain replies that he has full legal authority. This last thing is still bothering the Doctor as he stands beside the TARDIS watching the castle disappear and the Judoon spaceships lifting off. He decides to go to Balmoral to ask more about it but after landing the TARDIS near Balmoral and walking towards the castle he is disturbed to hear the sounds of full scale battle.
Standing on a hill he looks down on a siege. Troops have dug trenches and drawn up field guns. They are keeping up continuous fire at Judoon who stand in the open and return fire with hand blasters. He sees one human officer incinerated and a Judoon blown apart by a field gun shell before making his way down. He speaks to a sentry with such authority that he is immediately taken to see the commander, introducing himself as the envoy of the king. The General says that when his men arrived the castle was back in situ and that with a volley of field gun fire they destroyed some Judoon equipment, which the Doctor supposes was the force field generator. The Doctor tells him that the Judoon weapons are too strong and that a tactical retreat is needed so that he can speak to the alien Captain. When the Doctor wishes him well with his Boy Scout movement the General (now revealed as Lieutenant-General Baden-Powell) is astonished as he has only just started formulating plans for the foundation of the Scouts.
There is a rapid cease-fire allowing the Doctor to approach two Judoon sentries. They lead him to their Captain in his command post. He tells the Doctor that the Peacemakers have ordered the Doctor’s execution and pulls out his blaster. Ignoring this, the Doctor questions the validity of the Judoon presence and their authority to be interfering with Earth history. The Judoon says he is there on the orders of the Galactic Council and produces a document as proof. It bears the Council seal but the Doctor uses his sonic screwdriver to show that the wording of the document has been altered. In reality it ordered that there should be no outside influence on Earth’s destiny. The Captain asks the Doctor’s advice and is told to leave immediately. Enraged, the Captain orders his men to prepare for immediate departure and then says he needs to avenge the honour of his people.
The Doctor follows the Captain to the sitting room where the king, Martha and Carruthers are with the four Peacemakers. One asks why the Judoon are leaving and draws a weapon. The Captain is too quick for him and pulls out his own gun, incinerating three of the aliens. Challoner takes Martha hostage and produces a device which makes both of them disappear. The Doctor tells the Captain that he has his revenge but the Captain asks him to destroy Challoner to satisfy his honour and then promises that the Judoon will not set foot on Earth again.
As the Doctor races to the TARDIS the Judoon ships are taking off. He locks onto a signal from a Temporal Reversal Generator (too sophisticated for Challoner and therefore stolen) and lands in the laboratory. Challoner has tied up Martha and says he is about to destroy the major cities on Earth. He will recruit new mercenaries, perhaps Ogron or warriors from Schlangi, to do his dirty work. The Doctor tells him to switch off the generator before it malfunctions. When Challoner asks him why he thinks it will go wrong the Doctor responds by pointing his screwdriver at the console. It bursts into flame. Challoner points his gun at the Doctor but Martha, despite being tied hand and foot, launches herself at the alien and knocks him into the bowl of fiery liquid beneath the glowing sphere. The sphere explodes and the whole mechanism vanishes in what the Doctor calls a ‘temporal blowback…it abolished itself’.
Martha asks him if this means they can relax and have some gracious living but he reminds her of the Temporal Reversion Devices scattered around the world. He offers her a bomb-disposal team’s working holiday in Paris, Rome, Berlin, Tokyo, Moscow, Beijing…
Barely able to conceal her smile, Martha leads the way into the TARDIS.