In a well-furnished manor house, a figure in a sweater throttles one of the servants to death. The figure is tied up, kept under guard by a South American Indian with a plate in his lower lip. Elsewhere in the house, a young woman sleeps -- and she's the exact double of Nyssa...

The TARDIS materializes on the railway platform at Cranleigh Halt on 11 June 1925. The Doctor and his companions emerge to look around, and the Doctor tells Adric that he'd always wanted to drive a train when he was a boy. Outside the station, they are greeted by a chauffeur who calls the Doctor by name and seems to have been expecting his arrival. The chauffeur, Tanner, reacts strangely to the sight of Nyssa, but he soon recovers and explains that Lord Cranleigh decided to bat first to give the Doctor a chance to get here. The Doctor realizes that he's expected to participate in a cricket match and enthusiastically accepts. The game is already underway by the time they arrive, and when Lord Charles Cranleigh greets them he is also thrown by the sight of Nyssa -- who is the exact double of his fiancee. As he leads the Doctor to the cricket grounds the Doctor comes to realize that he's been mistaken for someone else who was expected by train, but he plays along with the case of mistaken identity in order to participate in the match.

Lady Cranleigh, Charles' mother, is just as astonished by the sight of Nyssa as the others, but despite her pardonable curiosity doesn't ask too many questions; when Nyssa tells her she comes from the Empire of Traken, Lady Cranleigh is too polite to admit she doesn't know where that is. The Doctor very nearly wins the game single-handedly, doing just as well bowling as he does at bat (although Nyssa and Adric are completely unable to follow what's going on). Charles is delighted and invites them all back to his home for the masked ball.

As the Doctor and his friends arrive at Cranleigh Hall, the Indian is watching them from a hidden room on the upper floor, as the man who killed the servant struggles to untie himself and escape. Cranleigh introduces the Doctor to Sir Robert Muir, the Chief Constable for the area, and his fiancee -- Ann Talbot, who is indeed Nyssa's exact double. The Cranleighs' servant Brewster mixes drinks for the visitors while Tegan admires a beautiful black orchid, a souvenir of Charles' elder brother George's expedition to the Orinoco. Unfortunately George never returned from his last expedition two years ago. Ann had been engaged to him, but she is now engaged to Charles.

Cranleigh shows his visitors to their rooms, and the Doctor studies the costume which has been chosen for him -- a Pierrot figure with a mask that completely covers the wearer's face. Meanwhile, Nyssa helps Tegan to dress in a faerie costume and Tegan demonstrates the Charleston, a period dance which she learned for a play in school. Ann arrives with two identical costumes which she and Nyssa can wear; the only way to tell them apart will be the mole on Ann's shoulder.

The man in the upper room escapes from his bonds, knocks out the Indian and flees. The Doctor is drawing a bath when he hears movement in his room; he emerges to find that a secret panel has opened in his wall, and enters, curious, only to be trapped in the passages when the panel swings shut and latches itself. Furious with himself, the Doctor starts searching for a way out, while in his room, the fugitive emerges from hiding and finds the Doctor's Pierrot costume -- and mask...

While the Doctor searches for a way out of the passages, his companions are having a fine time at the dance. Tegan dances with Sir Robert Muir and finally gets a chance to dance the Charleston. Ann and Nyssa dance together into the house, and when they emerge nobody is able to tell one from the other. Adric decides he isn't very good at dancing and sits out the dance, stuffing himself at the buffet. Lady Cranleigh abruptly departs when she sees the Indian, Dittar Latoni, standing on the sidelines; he warns her that his friend has escaped and that Digby has also disappeared, and, worried, she sets off to investigate.

The Doctor finds another panel and this time discovers the secret latch. The panel opens onto a part of the house which he doesn't recognize, and as he explores he finds closets full of men's clothing and books -- some botany texts and some in Portugese. He finds the upper room with the window panels but doesn't yet realize its significance. Searching for a way out, he opens a closet door -- and finds the body of the murdered servant stuffed inside...

The Doctor returns to the passages, where he meets Lady Cranleigh and Latoni. Their surprise is mutual, but the Doctor recovers and shows them the dead body. Lady Cranleigh identifies the man as a servant and asks the Doctor to keep this incident from the other guests. The Doctor reluctantly agrees to do so when she assures him that she will call the police afterwards. She and Latoni lead him back to his rooms, where he changes into his Pierrot costume -- which has been left just the way the killer found it...

Adric piles more food on his plate at the buffet, and when one of the doubles notices and calls him a pig, this is all the proof he needs that she's Nyssa. Tegan is starting to wonder where the Doctor is, but none of the companions notice the movement of servants around the sidelines of the party as the dead servant's body is discovered in the foyer.

The killer, a man with hideously mutilated features, has taken Ann back to his room, where he watches tenderly over her. He hides in the corner of the room when Latoni and Lady Cranleigh arrive and try to open the locked door; the sound awakens Ann, who flies into a panic and flees without having seen the hiding man. As Lady Cranleigh comforts the hysterical Ann, Latoni approaches the man with a length of rope...

The Doctor arrives in the foyer to find Charles and Sir Robert studying the body, and Ann's discarded mask lying nearby. At that moment Lady Cranleigh arrives with Ann, and Ann takes one look at the Doctor's costume and instantly accuses him of attacking her and killing James. The Doctor proclaims his innocence and demonstrates that anybody could have been beneath the mask she saw -- but Ann was in charge of the costumes and knows that the Doctor was the only Harlequin. Sir Robert demands that the Doctor account for himself, and the Doctor is forced to admit the truth about his identity -- the literally unbelievable truth.

Although he'd agreed for Lady Cranleigh's sake to remain silent, the Doctor has no choice but to tell Sir Robert about finding the other body in the secret passages... but Lady Cranleigh denies that it happened. The Doctor takes Sir Robert to see for himself, only to find that the body is gone, replaced by a large china doll. He is taken back to the sitting room to give a full account of himself, and Charles telephones his old friend "Smutty" about the man he'd sent for the cricket match -- only to find that the doctor they'd been expecting missed his train. Sir Robert arrests the Doctor on suspicion of murder, and Tegan, Nyssa and Adric are arrested as accomplices -- or accessories. They will face the same penalty as the Doctor if he is found guilty.

While on their way to the police station, the Doctor convinces Sergeant Markham to pull in at the railway platform so he can prove his story -- but once there he finds that the TARDIS is gone. Fortunately, it turns out to have been taken to the police station, and the Doctor allows Sir Robert and Sergeant Markham to enter and see the truth for themselves.

Back at Cranleigh Hall, Lady Cranleigh admits to Charles that the Doctor was telling the truth -- the servant Digby is also dead. She refuses to confess to the police, but Charles, realizing that things have gone too far, decides to admit everything. But first, he must tell the truth to Ann. Meanwhile, the killer silently struggles free of his bonds, creeps up behind the unwary Latoni and throttles him to death. As he dies Latoni slips the key of the room into a crack in the floor, and, unable to unlock the door, the killer piles newspapers beneath it and sets it on fire.

As Sir Robert tries to come to terms with the inside of the TARDIS and the Doctor's true identity, Constable Cummings enters with news from Cranleigh Hall. Once he's gotten over his shock he manages to explain that Lord Cranleigh has called to report the death of Digby. The Doctor offers to take Sir Robert and Sergeant Markham directly back to the Hall to investigate in person. They arrive just as Ann rushes out of the house, horrified by Charles' revelations. As they enter the Hall to confront Charles and his mother, the killer manages to burst through the burning door and arrives in the foyer, where he grabs Nyssa and flees upstairs with her. By now, the Doctor has guessed the significance of Dittar Latoni and the black orchid, and Lady Cranleigh confirms his suspicions -- the killer is her son George.

George drags Nyssa onto the roof, and as Charles tries to climb up the outside of the Hall, the Doctor searches for a way upstairs through the spreading fire. Lady Cranleigh explains to Sir Robert that George was captured and tortured by a tribe of South American Indians for stealing their sacred black orchid. Latoni, the chief of a rival tribe, rescued him and brought him back to England, but the experience had turned George's mind. Lady Cranleigh has tried to keep him safe in her home and care for him, but now he's committed murder...

Charles reaches the roof and tries to talk sense into his brother, but George panics and knocks him down as the Doctor arrives. The Doctor is more successful in calming George, and manages to convince him that he isn't holding Ann at all, but Nyssa. George sees Ann watching him from below, and eventually, reluctantly, lets Nyssa go. Charles, watching, rises to thank his brother for seeing reason, but George recoils from him -- and falls over the edge of the roof to his death.