The Doctor

It is 1813 and a ball is underway at a mansion in Bath, England. Two men are having an evening stroll in the gardens outside. Lord Galpin is angry with Lord Barton, declaring he has tarnished his sister's reputation but Lord Barton only mocks him back, remarking he should challenge him to a duel instead of accusing him. Lord Galpin suddenly stops and remarks that Lord Barton is an exciting man fun of scandal and affairs, remarking he wants to be him. Lord Galpin suddenly grabs Lord Barton, causing electricity to crackle between them. In a flash, Lord Barton's charred body falls to the ground as a corpse while Lord Galpin, who has now transformed into a copy of Lord Barton, leaves the scene.

Back in the ballroom, The Doctor and his travelling companion, Ruby Sunday, dance together among the other guests, both are able to dance in perfect synchronisation with the others thanks to the psychic earrings The Doctor has given her. Once the music ends, the hostess, the Duchess of Pemberton, congratulates them and asks Ruby to come and meet some suitors rather than remain dancing with The Doctor. Ruby follows the Duchess leaving The Doctor, who uses his sonic screwdriver to scan for alien technology. This leads him to notice a well-dressed man brooding on a balcony above the ballroom, observing rather than partaking in the event. The Doctor leaves the ballroom and approaches the man on the balcony and jokes with him about his brooding nature. When asked the man reveals that his name is Rogue. The Doctor asks why Rogue is not partaking in the celebrations and looks as if he is expecting trouble. In response Rogue asks The Doctor to follow him outside. He leaves and The Doctor follows him.

Elsewhere, Ruby is mingling with the Duchess when the copy of Lord Barton returns to the ballroom and begins flirting with Ruby. While initially impressed by his appearance, the man's words irritate Ruby, leading her to dismiss him, shocking the man. Ruby then notices a portrait of an old woman hanging on a wall - the woman in the portrait is identical to the tea lady at EMI Records, Penny Pepper-Bean and others that she and The Doctor have encountered throughout their travels. The Duchess approaches and explains the woman was the mother of the duke whose mansion this once was. The pair then notice Lord Barton trying to flirt with Lady Emily Beckett – who then leave the room together. Ruby chooses to follow them to the library while the Duchess pursues another patron outside. Outside, the Duchess spies one of her housekeepers nearby. Outraged that a woman of such low social standing would dare to be seen near the house during her party, she approaches and lambasts the woman. However, the housekeeper only replies that she hates being a member of her staff and wants to be the Duchess instead. She then grabs the Duchess, lightning flashes, and she is now a copy of the Duchess. Like what happened to Lord Barton earlier, she leaves the corpse of the Duchess in the garden to be discovered by The Doctor and Rogue.

Ruby Sunday

The Doctor soon assesses that she died from an alien intervention, noting that Rogue is unsurprised. Rogue then draws his gun, holding The Doctor at gun point he forces him to march to the outskirts of the garden. He reveals he is being paid to assassinate the Chuldur - a race of shape-shifting bird-like humanoids that is killing guests at the party to become them - and that he believes that The Doctor is that Chuldur. The Doctor tries to convince him otherwise as Rogue reveals his cloaked spaceship, parked a few steps from The Doctor's TARDIS.

Once inside his spaceship, Rogue traps The Doctor inside a triform, intending to incinerate him. Despite The Doctor’s argument that he is not a Chuldur, Rogue's scanner confirms The Doctor is a shapeshifter, so he still intends to kill The Doctor anyway. Thinking fast, The Doctor tosses his psychic paper on the scanner, causing it to reveal his Time Lord biology, by showing his numerous incarnations. This enables The Doctor to prove to Rogue that he is a Time Lord from Gallifrey and not a Chuldur. Realising that The Doctor is telling the truth Rogue lets him go. The Doctor then takes Rogue back to his TARDIS to use its console to reconfigure the triform so it will send the Chuldur to a random but isolated dimension in the universe, rather than kill them. Meanwhile, Rogue admires The Doctor's TARDIS, leading The Doctor to offer to take him traveling - but Rogue refuses stating that he did once travel with someone he cared deeply for, but then lost them during a terrible day. The pair move close to each other until the TARDIS dings - the reconfigured triform is ready.

Rogue

While this has been happening, Ruby has made friends with Emily, despite having interrupted Emily’s romantic liaison with the copy of Lord Barton in the library. Back in the ballroom, the copy of the Duchess meets with the copy of Lord Barton and they observe the dancers, discussing their plans. The Duchess remarks that she had hoped that a member of royalty would have appeared but she will settle for Ruby Sunday - the woman's personality making her interesting to her. Ruby and Emily, while wandering the corridors in the mansion, encounter a charred corpse. Worried, Ruby remarks they have to find The Doctor. They return to the ballroom where The Doctor deduces, from Ruby's remarks about how very Bridgerton the entire scenario is, that the Chuldur are taking the forms of people to cosplay as them, inspired by the show's drama and the scandals within the era it is loosely based upon. To draw the Chuldur out The Doctor suggests that he and Rogue dance, as that would be seen as ‘scandalous’.

As they dance, everyone in the room notices, the other patrons leaving the dancefloor as the two men dance together. Eventually, The Doctor breaks the dance up by starting an argument with Rogue, the pair soon create a scene resembling two lovers, a scandalous affair and ending in a marriage proposal from Rogue that The Doctor rejects, both running from the room. The entire scene grips the Chuldur's attention, and four of them transform into their natural avian forms and chase the pair outside.

Hiding in the roof space of a horse-drawn carriage, Rogue realises that he has wrongly thought that they only had to catch one Chuldur. Unable to locate The Doctor and Rogue the Chuldur end their pursuit and return to the ball. The Doctor and Rogue deduce that the Chuldur intend to cosplay across the country to London and then wage war against the world by becoming royalty and government leaders. The Doctor adjusts the triform to now contain six people, albeit stretching its power so now they only have one attempt to catch all four Chuldur.

Chuldur Duchess

Back inside, Ruby finds Emily has fled to the library, seemingly in shock at the Chuldur revealing themselves. Ruby tries to console Emily and chooses to reveal that she has come from the future. But to Ruby’s surprise Emily's is not shocked by this revelation and her demeanour changes as the woman grabs Ruby's hand, revealing that she is a Chuldur too and now she wants to cosplay as Ruby, as lightning flashes.

The rest of the Chuldur, having returned to the ballroom, remain in their alien form when the Chuldur, that was the Duchess, declares that a wedding must occur. Another of the Chuldur grabs a nearby vicar and forcefully kills him and turns into him to cosplay for the wedding. The Doctor and Rogue watch from the balcony above as the Chuldur that was Lord Barton enters from the other side, revealing Ruby by his side. Devastated, The Doctor leaves the balcony, believing Ruby has been killed; he remembers he once promised Carla Sunday he would keep Ruby safe. Embittered, The Doctor arrives in the ballroom and interrupts the cosplay wedding. He then activates the triform around the Chuldur wedding party, trapping them.

But as he is about to vanquish the Chuldur, including the Ruby copy, Ruby reveals that she is the real Ruby - having escaped the copy of Emily by using the psychic earrings to fight off the Chuldur. Ruby then pretended to be a Chuldur to discover what they are up to before The Doctor trapped her in the triform. The Chuldur, that was Emily, appears, her nose broken from where Ruby had hit her with a book. The Chuldur, that was Emily, charges at The Doctor only for Rogue to intervene and force her into the triform trap. Rogue orders The Doctor to press the button and send them away, but The Doctor breaks down as doing so will condemn Ruby to a lingering death, something he is unable to do. Realising The Doctor’s dilemma Rogue moves towards The Doctor and comforts him. However, on doing so, he takes the triform remote control from the distracted Doctor, before leaping onto the triform and pushes Ruby from it - so taking her place. He then presses the button resulting in himself and all five Chuldur disappearing into an unknown alternate dimension. However, just before Rogue disappears, he tells The Doctor to find him.

The following morning, having sent Rogue's spaceship into an orbit around the moon, The Doctor is sitting on some steps in the mansion’s garden. Discovering him there, Ruby suggests that they use the TARDIS to rescue Rogue but The Doctor rebukes her, stating there are endless number of dimensions and so finding Rogue is not possible, especially since he never even revealed his real name. Before they depart in the TARDIS, The Doctor takes out Rogue's ring from his pocket and puts it on his own finger.