March 1215: King John has paid an unexpected visit to the castle of Sir Ranulf Fitzwilliam, demanding more tribute for his Crusades. Fitzwilliam is unable to provide any more money -- he's already willingly given his entire fortune to the King -- and the King claims to be insulted. His champion, the French knight Sir Gilles Estram, challenges Fitzwilliam's honour, and Fitzwilliam is shocked when his hot-headed young son Hugh picks up the gage and agrees to face Estram on the morrow. The fourth of March dawns with the local serfs and Fitzwilliam's servants gathered to witness the jousting match, but the match is interrupted when the TARDIS materializes, startling everybody -- except the King.

The Doctor, Tegan and Turlough emerge from the TARDIS and are taken aback when the King loudly welcomes them as "his" demons and Estram seems to imply that the King has summoned aid from Lucifer so he can win the match. The King invites them to sit by him, and most everybody else shuffles back to avoid them as they take their seats. The match is rejoined and Estram easily defeats young Hugh, but as he approaches to deliver the killing blow, the Doctor begs the King to spare the young man's life. The King agrees to do so, much to Fitzwilliam's relief... but Hugh is shamed, feeling that he has been dishonoured.

The Doctor and his companions are welcomed into the castle, but Hugh, determined to learn who the new arrivals really are, kidnaps Turlough and takes him to the dungeons to be tortured. Sir Gilles has Ranulf's wife Isabella taken to the dungeons as a hostage to ensure his continued good behaviour, and the furious Ranulf decides to seek answers from the king's demons. The Doctor and Tegan are settling into the guest quarters, where the Doctor informs her that on 4 March 1215, King John was in London taking the Crusader's Oath -- and the date is too well documented to be in error. Fitzwilliam confronts the Doctor and Tegan, who try to assure him that they mean him no harm. The Doctor learns that Fitzwilliam's cousin, Sir Geoffrey de Lacy, was summoned to London a week ago by the King to take the Crusader's Oath, but the King hasn't mentioned him since he arrived. The Doctor suggests that the King may be an impostor, but Ranulf doesn't know what to make of this suggestion and isn't entirely convinced that the Doctor and Tegan are friendly...

In the dungeons, Sir Gilles finds Hugh preparing to place Turlough in an Iron Maiden, and has Hugh chained up along with his mother and Turlough. He then leaves the castle to arrange for the TARDIS to be brought inside its walls, and confronts a new arrival -- Sir Geoffrey de Lacy, who just left the King in London two days ago and is startled to hear Sir Gilles claim that the King is here. Sir Gilles accuses Sir Geoffrey of lying and has him arrested.

Tegan recognizes the Tissue Compression Eliminator and throws a knife at the Master's head, but he catches it in mid-flight. He gives the Doctor a choice of weapons -- the knife or the TCE? The Doctor takes the TCE, but the Master knows he will never use it. The King demands that the Doctor finish off "Sir Gilles", and when the Doctor refuses, orders the guards to put him in the Iron Maiden. The Doctor objects but the King demands blood -- either Sir Gilles or Sir Geoffrey. The Doctor has no choice but to stand by as the Master is placed in the Iron Maiden, the door is closed... and the Iron Maiden dematerializes with the sound of a TARDIS, revealing the Doctor's final mistake.

The King knights the Doctor as his new champion, and the Doctor's first act is to arrest Sir Geoffrey and take him to the dungeons -- to Ranulf's fury, as he'd just started to trust the Doctor. In fact, the Doctor just wants access to the Master's TARDIS, which he believes will be kept out of way in the dungeons. He's quite right; the Master has appeared and released Isabella and Hugh, claiming that he is attempting to stop the Doctor's evil schemes. The Doctor, Tegan and Geoffrey arrive and release Turlough, and the Doctor explains the truth to Geoffrey and asks for his help. They intend to use the TARDIS to take Geoffrey and the impostor to London, but before the Doctor leaves the dungeon he activates the TCE and affixes it to the Master's TARDIS.

The Master reunites Isabella and Hugh with a grateful Ranulf, who places his men-at-arms in the Master's charge. The Doctor and his companions discover that the Master has moved the Doctor's TARDIS, and Sir Geoffrey decides to ride to London alone rather than wait to find it. Turlough helps Geoffrey to get to a horse, but the Master is waiting, and as soon as Geoffrey has cleared the castle walls he is brought down by a crossbow bolt. Turlough is captured as soon as he re-enters the castle.

The Doctor and Tegan are captured and taken to the dining hall, where Ranulf shows them the TARDIS, found in the King's chambers, and accuses them of trying to kidnap the King. Tegan manages to get inside the TARDIS and dematerialize, hoping to get back to the King's chambers, but she is stuck in mid-transit when an alarm she doesn't recognize begins sounding on the console. The Doctor takes advantage of the distraction to escape, and eventually locates the King's chambers -- where the Master is standing proudly next to a gleaming metallic robot. This is Kamelion, a souvenir from Xeriphas -- an infinitely adaptable decoy capable of changing its form and personality depending upon the will of its operator. Disguised as King John, Kamelion will provoke a rebellion and topple the real King from his throne, thus robbing the world of Magna Carta, the foundation of parliamentary democracy. And now the Doctor has unwittingly helped the Master to prove that the King is in league with demons. From here the Master will go on to undermine the key civilisations of the Universe, spreading chaos and paving the way for his own rule...

Sir Geoffrey is brought into the castle, but dies before he can explain what happened. Ranulf takes Turlough to face the King and the Master, only to find the Doctor there as well. "King John" orders Ranulf to boil the Doctor alive in oil, and the Doctor, in response, attempts to use his own willpower to break Kamelion free of the Master's control. Ranulf, seeing the King in torment, is about to kill the Doctor, but at that moment Tegan thumps the TARDIS console in frustration and the TARDIS materializes in the King's chambers. The distraction enables the Doctor to overcome the Master's will, and Kamelion transforms into the image of Tegan. The Doctor bundles the confused robot into the TARDIS and Turlough breaks free of his startled guards and follows.