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.