Donna Meeting Agatha

The TARDIS takes The Doctor and Donna Noble to a manor house owned by Lady Eddison. The year they discover is 1926 and they are about to meet the world's most famous crime fiction novelist Agatha Christie – who has been invited to a dinner party hosted by Lady Eddison and her wheelchair bound husband, Colonel Hugh.

Inviting themselves to the party The Doctor realises that they have arrived on the very day that it is reported that Agatha Christie disappeared – only to reappear 10 days later in a hotel in Harrogate.

The Doctor is obviously curious about this unexplained event in Agatha’s life but before he can start to investigate the body of one of the other guests – Professor Peach – is found in the library by Lady Eddison's friend and companion Miss Chandrakala. The Doctor, aided by Donna and Agatha, deduce that he was murdered with a lead pipe. The Doctor though also deduces that the murderer isn't human when he discovers morphic residue on the floor while examining the scene.

Lady Eddison

Aided by Agatha, The Doctor interviews the guests while Donna goes looking for more clues. She investigates a locked room, which Lady Eddison has had kept locked for forty years. Curious as to why Donna gets Lady Eddison’s butler to unlock the door. Inside Donna is attacked by a giant wasp after tracing a buzzing sound to a window. She manages to scare it off by using a magnifying glass to amplify the sunlight coming through the window. The wasp escapes just as The Doctor arrives. They make chase but it somehow disappears before they can catch up with it – not before it kills Miss Chandrakala along the way.

Realising that they are involved in a real-life murder mystery The Doctor, Donna and Agatha mull over the evidence they have gathered so far. It is then that The Doctor is poisoned with cyanide – luckily though it is not as fatal to him, as it would be to a human, so giving him time to use a combination of ingredients accompanied with a shock (in the form of a kiss from Donna) to detoxify himself.

Reverend Golightly

Later at dinner The Doctor tries to discover who the alien murderer is by lacing the guests' dinner with pepper – which acts as an insecticide to wasps. This though fails when all the lights are blown out by a sudden gust of wind. When the lights are restored they not only discover that Lady Eddison's son has been murdered in the confusion but Lady Eddison's necklace, 'The Firestone,' has been stolen.

In the sitting room, The Doctor and Agatha are able to reveal several secrets about the remaining guests and hosts. First it is revealed that Robina Redmond is in fact a thief called the 'The Unicorn' who coveted the Firestone and stole it in the confusion. Then Colonel Hugh confesses that he is actually not wheelchair bound as he appears to be; he faked the condition to make sure Lady Eddison did not leave him.

The biggest revelation though is the reason for the locked room. It seems it was locked after Lady Eddison's used it after returning from India forty years previously to recover from what was thought to be a bout of malaria but was actually because she was made pregnant by an alien known as a Vespiform, who gave her the Firestone necklace. The necklace is psychically linked to her son, whom she had given up for adoption and never saw again. Her son is actually the Reverend Golightly, who had come to associate Agatha Christie's novels with the way the world must work because Lady Eddison had been reading one when his alien biology was awakened in a moment of anger, and had killed those who were working against him in the manner of one of her novels.

Robina and Roger

Reverend Golightly, enraged at being discovered, transforms himself into his wasp form and pursues Agatha who has snatched the Firestone. The Doctor and Donna follow after her and they catch up with her at a nearby lake, where Donna throws the necklace into the water. However, as he is linked to the gem Reverend Golightly follows it into the water and drowns.

Still linked to the necklace, Agatha also nearly dies as well, but Reverend Golightly chooses to release her as his last act. The trauma though causes her amnesia, and so The Doctor deposits her at a Harrogate Hotel ten days later, thus explaining her reported mysterious disappearance.