Paris, 1626. A swordfight breaks out in the Crown’s Courage alehouse when Morand, the captain of Cardinal Richelieu’s personal guard, impugns the honour of Queen Anne in the presence of Musketeers Francois Rouffet and Patrice Delmarre. Delmarre is wounded, but Morand’s lieutenant is killed. This is not the first such duel to have taken place, but King Louis is oblivious to the gathering storm. In the Louvre, he and Cardinal Richelieu are playing chess, but the King pays too much attention to taunting his opponent and not enough to the game itself. The King’s knight takes the Cardinal’s bishop -- but a pawn stands in the shadows, waiting to strike. The crown may have struck a blow against the church, but it’s left itself vulnerable doing so, and the word for that, as Richelieu smoothly points out, is “checkmate”.

The Doctor has promised to take the former Pharaoh Erimem to a new life at the Braxiatel Collection, but her pet cat is lying on the console and it claws at him whenever he tries to reach the navigational circuits. Peri will be sorry to see her new friend go, but it seems that Erimem will be staying with them for a while longer, as the TARDIS materialises not on asteroid KS159 but in the middle of 17th-century Paris. Perhaps even the TARDIS wants Erimem’s company. The Doctor decides to take a look around while he’s here, and once Peri and Erimem have changed into suitable clothing for the period, they set off to explore.

Elsewhere, lady-in-waiting Marie de Chevreuse meets her lover the Duke of Buckingham, who gives her a letter to pass on to another woman. He assures Marie that she is his one true love, and that once their plot is over there will be no need to hide -- but in fact, the misogynistic Duke is only using her as a pawn in his own game...

Richelieu prepares to take his leave of the King, as it seems their conversation is once again degenerating into a sniping match over the relative merits of the Cardinal’s guards and the King’s Musketeers. The debate is exacerbated when Morand arrives with Rouffet and Delmarre and accuses them of murder. When the Musketeers claim that they were defending the honour of Queen Anne, Louis takes them at their word and dismisses all charges. Furious, Richelieu and Morand storm out of the Louvre, and the amused King gives Rouffet and Delmarre a sovereign each for their troubles and dismisses them. He has a ball to prepare for -- and although he won’t admit it in public, he’s not entirely sure what his wife will be doing for the occasion...

As the Doctor, Peri and Erimem walk the streets, the Doctor explains that France is undergoing a cultural crisis, but that Cardinal Richelieu -- who in real life is not the villain painted by Alexandre Dumas -- has a vision to unite his country. Peri begins to get the impression that she’s being watched with almost superstitious awe, while Erimem, who as a former Pharaoh is used to that feeling, is simply fascinated by the sights and sounds of the city around her. Even something as commonplace as a glass window is fresh and new to her. The Doctor suggests that they visit the Louvre; though it’s still the royal palace, it’s breathtakingly beautiful from the outside. Sensing a lecture coming on, Peri offers to let the Doctor show his willing pupil Erimem around the city while she strikes out on her own. The Doctor is reluctant to split up, but Peri assures him that she can take care of herself for a while...

Richelieu is frustrated and furious with the King, but as their carriage clatters through the streets of Paris he spots someone far more interesting outside. Richelieu orders Morand to follow the woman, find out why she’s out on the streets alone, and report back directly to him. Meanwhile, Rouffet and Delmarre set off to spend their sovereigns at the alehouse, and pass Madame de Chevreuse on her way to the Louvre. She’s looking for the Queen, but she finds the King instead -- and he realises that she’s brought a letter from one of the Queen’s suitors. The King informs her that the Queen has not returned from Lyon and demands that she hand over the letter; however, he is distracted by a footman and Marie slips away. However, as the Queen is not present to receive the message, it seems that Buckingham will have a long wait.

Peri is taking the air in a quiet, peaceful street when a man she’s never seen before, the Duke of Buckingham, approaches and greets her as if they were old friends... or perhaps more than friends. As Peri tries to put down what she assumes is an odd pick-up line, men dressed in the livery of the Cardinal’s guards arrive and attack her. Buckingham gallantly tries to hold them off, but is wounded and falls, and the men kidnap Peri. As she struggles to escape, Morand and his men arrive and try to rescue her, but the kidnappers shoot at them and retreat, taking the struggling Peri with them...

While Delmarre and Rouffet gaze in admiration at the Queen’s beauty, the Doctor and Erimem try to slip away to contact the King for help. The Doctor recognised the uniforms worn by Peri’s kidnappers, but though it seems obvious that she was mistaken for Anne, why would Cardinal Richelieu’s guards wish to kidnap the Queen? Delmarre tracks down the Doctor and attempts to re-engage him, scoffing at his claim that he needs to speak to the King on a matter of national security; even if this were true, the King is preparing for an important state ball and will speak to no one. Erimem, incensed by this challenge to her friend’s honour, challenges the Musketeers to prove their own honour and integrity by taking them to the King. The Musketeers gallantly agree to do as the lady requests.

Peri’s abductors gag her and take her to a deserted chateau some distance from the city. Upon arriving, Peri kicks her captor in the most painful place she can reach and makes a run for it -- but she is recaptured by Buckingham, who was not wounded by his attackers at all. These are in fact Buckingham’s men, and he orders them to lock Peri up and then dispose of their fake uniforms. Peri has no idea why Buckingham refers to her as “your majesty” and seems to think that they’ve been an item in the past; for his part, Buckingham dismisses her strange accent as a foolish jest at his expense, and locks her in a cell in the chateau’s dungeons, refusing to listen to her protests.

The real Queen Anne arrives at the Louvre to find her husband micro-managing preparations for the state ball. She is weary of his childishness, and he’s infuriated by her contempt and her claim that Richelieu holds the real power in France -- and, following his encounter with Madame de Chevreuse, he’s convinced that Anne is still seeing her lover, the Duke of Buckingham. As the argument escalates, the Doctor arrives with Delmarre and Rouffet, but the King dismisses the bizarre newcomer’s story -- until Erimem regally introduces herself as the Princess Erimem of Karnak. Her bearing and confidence save the day once more, convincing the King that even if he hasn’t actually invited her, this was merely an oversight on his part.

Erimem introduces the Doctor as her vizier, and the Doctor finally explains what’s happened. Richelieu then arrives in response to Morand’s report, and is quite surprised to find that the Queen is in fact alive and well. Morand confirms the Doctor’s suspicions that the kidnappers were not really Cardinal’s guards, but when he claims to have seen Peri in the company of the Duke of Buckingham, the King immediately concludes that his suspicions about the Queen are correct. As they argue, the Doctor explains to the puzzled Erimem that George Villiers, the Duke of Buckingham and the Prime Minister of England, may have had an affair with Queen Anne in the past. He’s more concerned with the state ball Louis has planned, convinced that the timing of the kidnapping is no coincidence.

Richelieu warns the King that news of the alleged kidnapping has spread surprisingly quickly, and that the Musketeers are after the blood of the Cardinal’s guards. However, since Anne is patently still alive and well, the King has already grown bored of the accusations and agrees to let the Doctor investigate on his own. Rouffet and Delmarre take the Doctor and Erimem out to speak with the informer “Blind Maurice,” a beggar who sees everything that takes place in the city. The Doctor bribes him with a bag of gold dust, and Maurice promptly directs them to an abandoned chateau outside the city which has seen suspicious activity recently. The Doctor decides to investigate with Delmarre and Rouffet, but asks Erimem to stay behind. This is partly for appearances’ sake -- no vizier should allow his princess to ride into danger -- but partly because he needs someone to keep an eye on Louis and ensure things don’t get out of hand in the city while he’s gone.

Buckingham’s plans are nearly complete, or so he thinks until a footman delivers an urgent message from Marie de Chevreuse warning him that the woman he has kidnapped is not the Queen at all. Enraged, Buckingham storms off to confront Peri, only to find that she’s broken out the glass in her cell’s window and escaped. Outside, the Doctor, Delmarre and Rouffet arrive to find that the “abandoned” chateau is crawling with soldiers -- and in the Doctor’s haste to find Peri, he slips up and is captured. Delmarre and Rouffet remain in hiding as the Doctor is taken away for questioning, deciding to wait for a chance to rescue both the Doctor and Peri at the same time -- or perhaps just run away, if that’s wiser.

Back in Paris, Queen Anne speaks with Madame de Chevreuse, who suggests that the Queen might be wise to rid herself of her foolish husband and then deal with Richelieu as she sees fit. The Queen decides to speak with Erimem, seeking a fresh perspective and wishing to learn more about the Doctor. Surprised to learn that Erimem places total trust in the Doctor, the Queen admits candidly that she trusts no one herself -- particularly not Marie, whom she keeps close only to keep an eye on her and stay up to speed on court gossip. The only person she trusts less than Marie is the Cardinal. Recalling her own experiences with corrupt priests, Erimem begins to wonder whether the Doctor was right about Richelieu. Perhaps he is involved in the kidnapping attempt after all.

Richelieu is certainly angry with the King, who cares more about the state ball than the chaos breaking out on the streets. Their argument soon degenerates into petty squabbling, and things get no better when the Queen arrives. Richelieu despairs that the King’s childish tantrums are threatening everything they’ve worked for, but the King remains confident that the so-called riots are merely the acts of a few hot-heads, and insists that the ball will unify the country -- in Louis’ name, not that of the Church. Richelieu storms out and heads back to his own palace, apparently unaware when Erimem stows away on his carriage to keep an eye on him.

Peri avoids the soldiers and slips into the stables, where she runs into Delmarre and Rouffet and learns that the Doctor has been captured looking for her. Meanwhile, the Doctor is brought before Buckingham, who reveals that his men saw the Doctor arrive with two Musketeers. Soon his friends will be captured, and in the meantime, Buckingham intends to find out exactly who the Doctor is and why he’s interfering in Buckingham’s plans.

Erimem recovers from the effect of the explosion, which is unlike anything she’s ever seen before. Morand is dead and several more of the Cardinal’s guards are injured, and the enraged Cardinal is convinced that the bomb was planted by Musketeers seeking revenge for the alleged kidnapping. He sets off back to the Louvre, and allows Erimem to accompany him; he still doesn’t know who she really is, but now believes that she is acting with honourable intentions. He hopes desperately to make Louis see sense, but it may be too late; as his carriage passes through the streets, he and Erimem see fights breaking out on every corner as the Musketeers and Richelieu’s guards have at each other.

Peri compares stories with Delmarre and Rouffet and insists upon rescuing the Doctor. Despite the odds, Delmarre and Rouffet reluctantly accompany her back to the chateau, but find every entrance guarded by Buckingham’s men. Fortunately, they are busy with their preparations, and Delmarre and Rouffet overpower three soldiers and seize their uniforms. Suitably disguised, Peri and the Musketeers scout out the chateau, but wonder uneasily what so many well-trained soldiers are doing within a stone’s throw of Paris.

Inside the chateau, Buckingham interrogates the Doctor, convinced that he’s a spy of a foreign power planning an invasion of England. He won’t take no for an answer, and when the Doctor continues to insist that his arrival is mere coincidence, Buckingham sets to work with the implements of torture left behind by the chateau’s previous owner. One such device hoists the victim by the wrists while his hands are tied behind his back. When Buckingham is finished, the Doctor has told him useful -- but he’s satisfied that the Doctor will tell nobody else anything at all, ever again.

Once again, the King and the Cardinal of France are at an impasse; Richelieu demands that the criminals be punished, but the King refuses to believe that his men would plant a bomb in the Cardinal’s palace. He flatly refuses to call off the ball or arrest his own men, believing this would send the wrong message to the dignitaries of France -- as if a capital city torn apart by violence isn’t a disturbing message by itself. He takes his leave, casually ordering Richelieu to change out of his filthy robes -- robes stained by the blood of Richelieu’s men. Richelieu begs Anne to appeal to the King’s common sense; even here in the Louvre they can hear the sounds of violence on the street outside, and it’s obvious that this is no mere brawl which will blow over, as the King seems to think. Marie de Chevreuse scoffs at Richelieu’s fears, but the Queen sends her out before Richelieu loses his temper. However, she can do nothing about the King, who still hasn’t forgiven her for her affair with Buckingham or her inability to bear him an heir.

While helping Erimem to dress, Marie gossips freely about her betters; she believes that the Cardinal lusts after Anne, and that the King and Cardinal are fools for dreaming of a united France. Erimem firmly puts the lady-in-waiting in her place, and finishes dressing for the ball in silence. However, when she emerges from the guest room she finds that matters have grown even worse. Realising that reason has failed, Richelieu is turning to threats; if the King does not call off the state ball and bring the criminal Musketeers to justice, Richelieu will have him excommunicated from the Catholic Church. Enraged by this insolence, King Louis orders his guards to place the Cardinal under arrest, despite the Cardinal’s threat that this may plunge the country into a religious war. Queen Anne loses her temper with her foolish husband and storms back to her quarters, and Erimem accompanies her, realising that this time the King really has gone too far.

The King pushes his way into Queen Anne’s chambers to demand that she attend the ball with him, and she loses her temper, ordering him out. Erimem is shocked by her hosts’ childish behaviour and calls them to task for it, but while Anne calms down and accepts her young guest’s wisdom, the furious Louis informs Erimem that after the ball she will be escorted out of the palace and thanked never to return to France. He also informs Anne that if she does not attend the ball, she will find herself sharing the Cardinal’s new accommodations. Anne realises that she has little choice but to attend the ball in order to stop the King from doing anything else rash, and Erimem, realising that she can do no more on her own, decides to fetch the Doctor.

The Doctor and his friends arrive in Paris to find bodies littering the streets. Everything is going as Buckingham had planned; the hoax kidnapping has set the factions of Paris fighting one another, and when the English army arrives they’ll be able to march straight through to the Louvre without opposition. Erimem finds the Doctor in the heart of the chaos, as she’d assumed she would, but after a brief reunion they must go their separate ways again. The Doctor and Peri must return to the Louvre to warn Louis and Richelieu of the danger, while Erimem, whom the Doctor has seen in battle, helps Delmarre and Rouffet to knock some sense into the French soldiers and ensure that they’re ready to fight when Buckingham arrives.

The ball appears to be going well, and the guests accept Queen Anne’s claim that the Cardinal is indisposed with a head cold. It seems that the King has successfully united the country under his own name, finally showing Anne’s precious Duke of Buckingham who’s really in charge here. Anne is somewhat taken aback, and privately rather pleased, to realise that, all along, the King has just been trying to show her that he can be the strong ruler he believes she wants him to be. But both are unaware that Buckingham’s army is now within sight of Paris, storming towards victory and an English France.

Unfortunately, without Erimem at his side the Doctor is unable to convince the guards at the Louvre to let him. Luckily, he knows a secret way in through the storm drains. Peri isn’t sure why they’re doing this when history dictates that Buckingham will fail, but trusts the Doctor when he assures her they must participate in history to ensure that it unfolds properly. They successfully break into the dungeons of the Louvre, and hear Richelieu shouting in the distance, vowing revenge. Peri distracts the guard, whom the Doctor knocks out, and together they release the Cardinal. It’s time the King was made to see sense.

Two Musketeers and a former Egyptian pharaoh can do little to stop the sheer chaos breaking out in the streets of Paris, until Erimem recalls the explosion she witnessed earlier. Following her instructions, Delmarre and Rouffet fetch explosives from their armoury and set off a very loud bang in the middle of the street, getting everyone’s attention. Erimem then lays into the soldiers, berating both sides for their shameful display and informing them that they’ve been tricked into leaving their city defenceless. Soon all of the soldiers, swayed by her words, vow to take up arms with her to defend Paris in the name of the both the King and the Cardinal -- of the Church and the Crown. When Buckingham rides up to the city gates, he finds them silent and inviting -- until the Musketeers and the Cardinal’s guards step out of hiding to reveal that his private army is surrounded. But Buckingham has come too far to turn back now, and he orders his men to charge. Battle is joined.

The Doctor, Peri and Richelieu burst into the state ball with dire warnings which the King is still fully prepared to ignore. However, he can no longer afford to do so, and this time the Doctor can warn him of the specific danger -- Buckingham has smuggled an army into France, a bit at a time, and thanks to the state ball the entire French aristocracy could fall before him. Louis’ instinctive reaction is to assume Buckingham has come for Anne, but the Doctor cuts through his jealousy to warn him that Buckingham could conquer France and use it as the spearhead for an entire European English empire. The King, finally realising the truth, puts right his mistakes by ordering that the ball be called off and the palace sealed.

Erimem leads the combined forces of the Church and the Crown into battle against the English, and the easy victory Buckingham had expected becomes a rout. Enraged, Buckingham fights his way to Erimem’s side and tries to take her hostage, but it’s obvious that he’s lost the battle; his men are deserting and his hostage is entirely capable of fighting him off herself. He thus flings her aside and rushes off, and Erimem, Delmarre and Rouffet set off in pursuit, leaving the battle to the soldiers. They lose Buckingham in the streets, but run into Maurice, who confirms that the man they’re chasing ran past him towards the Louvre. There, Buckingham kills the guard on duty and enters the palace, seeking revenge...

As Richelieu and the King try to calm down the unsettled aristocrats, Queen Anne prepares to retreat to her quarters, but Marie convinces her that she will be safer here. Peri notes that the Doctor is growing concerned for Erimem’s safety, and realises that he’s becoming attached to her. Buckingham then bursts in and holds a gun to the King’s head, and when Richelieu tries to intervene, Marie holds him at gunpoint as well. She’s been part of Buckingham’s plan all along, encouraging his affair with Queen Anne and helping to set up her kidnapping. The Doctor distracts Marie by throwing a sword at her beloved Buckingham, and Peri overpowers her and knocks the gun from her hand. Buckingham tries to kill the Doctor, but King Louis tosses the Doctor a sword, and swashbuckling commences. Erimem, Delmarre and Rouffet arrive just in time to see the Doctor pin Buckingham to the wall like a butterfly, and Delmarre realises that he’s lucky he never fought that duel with the Doctor after all.

Buckingham’s army is in disarray, and the Doctor produces a letter he found in the chateau, written proof of Buckingham’s intentions. However, Richelieu tears up the letter, knowing that this scandal could result in war with England and political chaos in Europe. Erimem, still of her era in many ways, believes that Buckingham should be executed for his crimes, but Richelieu suggests instead that he merely be exiled from France on pain of death and that King Charles of England should decide his punishment. Marie is also expelled from the palace for her treachery -- and she has it much worse, for Buckingham coldly rejects her, leaving her with nowhere to turn to. Peri is upset that Buckingham will get away with his crimes, but the Doctor assures her that he’ll meet his comeuppance in two years’ time at the hands of an angry Puritans.