The TARDIS arrives on Earth in 1925 where, due to a case of mistaken identity, the Doctor ends up playing in a local cricket match. The travellers then accept an invitation to a masked fancy dress ball, but events take on a more sinister tone as a number of murders are perpetrated at the country home of their host Lord Cranleigh.
The Doctor comes under suspicion but the murderer is eventually revealed to be Cranleigh's brother George, who has been kept locked up in the house ever since returning in a deranged and disfigured state from an expedition up the Amazon to find the legendary black orchid. George mistakes Nyssa for his former fiance Ann Talbot, who looks identical, and takes her up to the roof of the house. He is persuaded by the Doctor to let her go, but then falls from the roof to his death.