In an English country house two figures struggle before one of them falls dead. A young woman sleeps as a figure enters her room. The figure is then seen tied to the bed guarded by a South American Indian.
It is June 11, 1925, and as a train departs Cranleigh Halt railway station the TARDIS materialises. The crew encounters Lord Cranleigh's chauffeur, who has been expecting "the Doctor". Lord Cranleigh asks them to stay until the annual ball and offers them costumes. They are introduced to Ann Talbot, Lord Cranleigh's fiancée, who looks identical to Nyssa.
When Tegan admires a black flower, Lady Cranleigh explains it is a Black Orchid and was found on the Orinoco by her son George. Tegan recognises the name as George Cranleigh, a famous botanist and explorer. Lady Cranleigh says that George never returned from his expedition into the Brazilian forests. Ann was engaged to George.
The bound figure struggles against his bonds. When the Indian inspects the figure, he is knocked unconscious.
The Doctor picks a Harlequin outfit to wear to the ball. Ann comes to their room, presenting Nyssa with a dress identical to her own; the ball attendees will be unable to tell them apart.
As the Doctor prepares for the ball, a figure enters his room from a secret passage. Hearing a noise, the Doctor returns to the room, but only sees the opening. He enters the secret passage, but the panel closes behind him. The figure takes the Harlequin costume.
The ball has started. Lady Cranleigh talks with the Indian. He tells her his "friend" has escaped. The figure wearing the Harlequin costume dances with Ann.
The Doctor enters a room full of botany textbooks, finding the secret room where the figure was bound, a book written in Portuguese, and a corpse. The Harlequin enters the building with Ann. Ann tells it they should return to the party, but it grabs her by the wrist. Ann screams for help and a butler rushes to her assistance. The Harlequin strangles him, causing Ann to faint.
The Doctor returns to the secret room, finding Lady Cranleigh and the Indian, whom she introduces as Latoni — a Brazilian friend. The Doctor shows them the corpse, which she identifies as one of the servants. She requests he not inform the guests. The figure returns the Harlequin costume to the Doctor's room, then goes to a room where Ann is lying, and a hideously deformed face is revealed. Ann awakes and seeing the figure flees into the room where Lady Cranleigh and Latoni are. Latoni gathers rope, advancing on the deformed figure.
The servants inform Lord Cranleigh of events inside the house. He finds the dead butler and Ann's mask. The Doctor arrives wearing the Harlequin costume and Ann identifies him as her attacker. Ann implores Sir Robert to arrest the Doctor, and Sir Robert asks the remaining guests to go home. The Doctor proclaims his innocence, suggesting that someone else has an identical costume. Ann states there was only one Harlequin. He looks to Lady Cranleigh to provide an alibi but she stays silent. Sir Robert questions the Doctor regarding his identity, and he says he is a Time Lord and that he travels in time and space. Looking to Lady Cranleigh he mentions the other body, but she denies seeing it.
Showing Sir Robert the cupboard, the body has been replaced with a doll. Lord Cranleigh receives a telephone call from his friend "Smutty" Thomas who he thinks sent the Doctor to meet him, and he realises it is the wrong man. Lord Cranleigh informs Sir Robert that the Doctor is an impostor; the real doctor missed his train. The Doctor is arrested for murder, his companions accused of being accessories. They are driven to the police station. The Doctor asks the sergeant to divert to the railway station to show Sir Robert the TARDIS, but it is not on the platform. They find the TARDIS at the police station.
Lady Cranleigh tells Lord Cranleigh about the other body, that of Digby the servant, establishing his innocence. In the secret room, the bound figure slips his ropes and kills Latoni, after Latoni has hidden the room key. Unable to find the key, the figure starts stuffing newspapers under the door, then sets them on fire.
The Doctor invites Sir Robert and the police sergeant into the TARDIS. Astounded by what he sees, Sir Robert offers the Doctor an apology, but is still concerned about the murder. Lord Cranleigh telephones the police station, informing them of the second body. The Doctor uses the TARDIS, returning them to Cranleigh Hall. The secret room is ablaze with the fire started by the deformed figure, who breaks out and goes to the main hall where Lord and Lady Cranleigh are talking. The figure grabs hold of Nyssa and drags her upstairs. The Doctor cannot follow because the fire has spread. Sir Robert demands to know what the deformed figure is, and Lady Cranleigh reveals it is George. She insists George would not harm Ann, but the Doctor points out he has the wrong girl.
George carries Nyssa onto the roof. The Doctor asks Lord Cranleigh to distract George while he finds a way to their position. Lady Cranleigh confesses to Sir Robert: George's injuries were caused by Indians, who removed his tongue because they held the Black Orchid sacred. Losing his mind, he was rescued by another tribe of Indians, of which Latoni was a member. She admits that George killed Digby. Lord Cranleigh climbs onto the roof to confront George, and the Doctor also reaches the roof. The Doctor implores him to release Nyssa, telling him to look down and see Ann. George releases Nyssa. Charles approaches his brother to thank him, but George recoils and falls off the roof, dying.
Before the Doctor departs Ann gives Tegan and Nyssa their costumes as presents, and Lady Cranleigh presents the Doctor with a copy of George's book.