Donna returns to the TARDIS to change into a cocktail dress and together with the Doctor she arrives at the party outside, where Lady Eddison greets them. They use the Time Lord’s psychic paper to bluff an invitation and talk with the hostess about a recent chain of jewel robberies, instigated by a thief known as the ‘Unicorn’.
Lady Eddison’s husband – the wheelchair-bound Colonel Curbishley – and her son Roger arrive to join the party, followed close behind by Robina Redmond (a popular socialite), Reverend Arnold Golightly (whom Lady Eddison claims apprehended two thieves robbing his church the previous Thursday), and the party’s special guest – Agatha Christie.
The Doctor and Donna are thrilled to meet the famous author, who impresses them with some subtle artful deduction. Lady Eddison congratulates the writer on the success of her books and asks where her husband is. Agatha retorts that she can socialise without him by her side them resumes conversation with the rest of the guests.
After Lady Eddison asks her maid Miss Chandrakala to fetch Professor Peach from the library the Doctor pulls Donna aside to show her a nearby newspaper he has found – the date on which corresponds with the day Agatha Christie famously disappeared. Whilst Miss Chandrakala enters the house and opens the door to the library he explains to his friend that Agatha disappeared shortly after discovering her husband was having an affair. Her car was found the following morning beside a lake, and ten days later Christie was found in a hotel in Harrogate. She never spoke of the incident, but whatever caused her to disappear is soon to take place…
Suddenly Miss Chandrakala arrives, claiming that Professor Peach has been murdered. The Doctor hurries inside to the library where he discovers the victim was hit in the head with a lead pipe. As he rifles through some nearby papers he notices Agatha, who has joined him along with Donna, pick up a small piece of paper beside the fireplace.
He doesn’t mention this and instead continues searching. When the other guests arrive he again uses the psychic paper to deceive them – claiming he is an Inspector. Agatha hurries the guests to the sitting room whilst the Doctor examines the floor of the room, where he discovers traces of Morphic Residue, which is left behind when certain alien species change their shapes – one of the guests downstairs is an alien in human form.
He and Donna hurry away where Agatha meets them. The Doctor asks her to accompany him as he interrogates the other partygoers and then hands Donna a spyglass and asks her to search the rest of the house for more clues. The Time Lord enthuses about solving a murder mystery with a famous crime author but Agatha is not so jubilant. Disturbed by his easy-going attitude to murder she warns him that her participation in the matter is for the sake of justice, not his amusement.
They go to the sitting room where they question each of the other guests in turn, looking for an alibi. Reverend Golightly claims he was unpacking his cases in his room, Robert tells them he was strolling the grounds alone (when in fact he was engaging in a secret liaison with a serving boy), Robina recalls going to the bathroom before preparing herself for the party (which unbeknownst to the detectives involved loading a pistol concealed in her purse), the Colonel maintains that he was inspecting some old war journals (in reality a selection of risqué magazines) and Lady Eddison claims she was taking afternoon tea (a cover-up for an indiscreet sip from a hipflask).
With all the guests apparently busy at the time of the murder the Doctor and Agatha ponder the mystery together. After the Doctor slips into a recollection of the time he visited Belgium Agatha reveals the paper she took from the library. What is left of the burnt document is a single word – ‘maiden’.
Upstairs Donna continues checking the other rooms. She is met by Greeves the butler, who begrudgingly unlocks one of the sealed rooms for her. He explains that when Lady Eddison returned from a trip to India many years ago she caught malaria, and locked herself away in the room until she was well again. Forty years later the room has remained sealed.
Greeves leaves Donna to investigate but she finds very little. After a few moments she hears a buzzing noise coming from the window. She opens the drawn curtains and is shocked to discover a wasp the size of a human hovering outside. It smashes the window and flies at her, but she manages to stall it by using her magnifying glass to concentrate the sunlight outside onto its chest. She runs out into the hall and slams the door shut, all the while calling for the Doctor to help her.
The wasp implants its sting into the door as the Doctor and Agatha arrive. They hurry inside the room only to discover the creature has gone; only its sting remains. Agatha is sceptical about the existence of such a creature but the Doctor insists it may hold the key to solving Professor Peach’s murder.
Down in the kitchens the servants discuss the recent goings on. After a few moments Miss Chandrakala ponders what the Professor might have been looking for in the library and hurries away to talk with Lady Eddison. She runs outside but is distracted when one of the stone gargoyles perching on the roof of the building begins to move. Suddenly it topples from its resting place and plummets towards her. She screams out but is soon silenced…
Hearing the scream the Doctor, Donna and Agatha hurry outside. They find Miss Chandrakala, who has time to mutter the words, “the poor little child” before breathing her last. Once again a buzzing sound is heard and the Doctor looks up to see the giant wasp hovering above them. It flies off in the direction of the house and the Time Lord follows in hot pursuit. He and his friends arrive upstairs to discover the creature waiting for them.
Having seen the creature for herself Agatha is dumbstruck. Suddenly it tries to attack again but misses and is chased off by Donna, who threatens it with her spyglass. The Doctor insists they must follow the creature and they run to chase it once again before it takes human form once again. The Doctor orders the creature to show itself but his plan is foiled when all of the other guests appear to answer his call.
Some time later Roger comforts his distraught mother whilst the Doctor ponders Miss Chandrakala’s last words. Lady Eddison and the other guests turn to Agatha for an answer to the mystery surrounding the house but she insists she is unable to help – the Doctor is the man with the answers.
A while after Donna finds Agatha sitting alone outside the house. They discuss Agatha’s books (which she believes will not be remembered) and then their partners, both having lost a loved one in the past. Suddenly Agatha notices one of the nearby flowerbeds has been disrupted and upon examining them discovers a wooden box lying in the undergrowth. She and Donna take it inside where the Doctor opens it and finds it full of tools a thief would use to commit a burglary. As Agatha realises the Unicorn may be close by Greeves brings in a tray of drinks and the three friends discuss the giant wasp, which the Doctor explains is actually called a Vespiform. He ponders why the creature appears to be behaving like a character in one of Agatha’s books but is silenced when he realises something is wrong with his drink. He lurches forward and tells the others he has been poisoned. He staggers to his feet and after detecting traces o cyanide in his glass, Agatha runs after him with Donna alongside.
The Time Lord, increasingly coming under the affect of the poison, makes his way to the kitchen where he begins looking for something to save his life. After downing a bottle of ginger beer he tells Donna and Agatha that he must consume something with high protein content. Donna fetches some walnuts and he begins hurriedly consuming them. Unable to speak with his mouth full he mimes to his companions the ingredient he needs next to combat the cyanide. A botched game of charades ensues and after some effort the Doctor asks for something with a lot of salt – which Agatha provides by handing him a jar of anchovies. He then asks them to give him a shock so as to complete the ritual and Donna obliges by kissing him on the lips. Suddenly he lurches back and exhales a cloud of white powder – the poison has gone.
That night the guests all sit down to dinner and the Doctor ponders the day’s events – including the failed attempt on his life. After revealing that any one of the guests could have spiked his drink he reveals that he too has turned his hand to poison – he has laced the soup the guests are consuming with pepper. He claims the active ingredient of pepper is Piperine, which is also used in insecticides. He waits to see which guest this will have an effect on, hoping it will reveal the identity of the masked Vespiform.
However, before he finds an answer the lights go out and the party is plunged into darkness. A buzzing sound fills the room and the party is thrown into disarray. When the lights return the Vespiform is gone, but it is not the only thing that has. Lady Eddison’s necklace – the firestone – has been taken and her son Roger is dead – impaled with a knife.
As the night wares on the Doctor, Donna and Agatha retire to another room and discuss the two new crimes. Donna explains that Lady Eddison brought the firestone back to England when she returned from India, and it is now worth a great deal of money. The Doctor, on the other hand, is more concerned with the motives of the Vespiform. He turns to Agatha for answers, claiming that she is the only person who can solve the murder. She refutes his claims but he insists that she is able to help – her murder mystery stories are the best ever written, the reason for this being that she understands human nature better than anyone else because she has encountered the many facets of it throughout her life, and thereby can construct plausible motive for the villains in her books.
Once Agatha has had time to think the Doctor calls the remaining guests to the study, where he introduces Agatha. She begins pondering the situation and first turns her attentions to Miss Redmond, whom she claims is an impostor assuming someone else’s name. She recalls how Robina claimed she visited the toilet that afternoon then reveals that the box of tools she and Donna found was beneath her bathroom window. She accuses her of trying to dispose of the box when Donna began searching the house following the murder. She opens the box and shows the guests the tools inside, then reveals that Robina Redmond is in fact the ‘Unicorn’.
Robina confesses that Agatha is correct, and produces the missing firestone from her dress. She hands it to the Doctor and he examines it, whilst Donna questions if the thief is also the killer. Agatha and Robina claim this is not so, and the finger of blame appears to fall upon the Colonel instead.
Admitting to being court out he reveals his secret – he is in fact able to walk. This stuns Lady Eddison, who listens as he explains that the reason for his deceit was to garner her sympathy so that she would not leave him for a younger man. He asks Agatha how she knew he was lying and she confesses she did not – her intention of addressing him was to declare his innocence in the matter.
The Colonel sits back in his chair and Agatha turns to Lady Eddison, questioning her about the firestone jewel. She reveals that Eddison was not suffering from malaria when she returned from India, she was in fact pregnant. Unmarried at the time, she locked herself away until the baby was born – the only person aware of this being Miss Chandrakala (hence her final words). Lady Eddison confesses that she had the child given away for fear of being ill thought of by society then, prompted by the Doctor’s claim that her pregnancy was not an ordinary one, tells the story of how she came to bare a child.
When she was in India she witnessed a dazzling light falling in the sky. The next day she met a young gentleman named Christopher, with whom she fell in love. He revealed to he that he was a shape-shifting alien who had come to Earth to learn about human beings. Because she loved him she did not regard this as a problem and the two continued their relationship until Christopher – alias a Vespiform – was killed in a monsoon. Before he died he left her the firestone, and not long after she discovered he was pregnant with his child – also a Vespiform.
Agatha continues the story, claiming that Miss Chandrakala took the child to an orphanage. Forty years later Professor Peach found the child’s birth certificate in Lady Eddison’s library (the document found by Agatha – the word “maiden” referred to the maiden name of the child’s mother) and when Miss Chandrakala considered what he might have uncovered she rushed to warn her mistress, only to be killed before she could reveal what was going on.
Donna assumes Lady Eddison to be the killer, protecting her alien child, but Agatha insists she is innocent. The Doctor then assumes the mantle of detective and asks Lady Eddison what she was doing the previous Thursday night. She claims she was alone in her library reading one of Agatha Christie’s books then asks the Doctor why this is important. He explains that something significant happened that night – the break-in at the church. Turning to Reverend Golightly he reveals that the vicar is in fact Lady Eddison’s child. He then guesses that the night of the burglary, when he caught the two thieves, the anger he felt at the moment allowed him to access his Vespiform heritage for the first time.
He shape-shifted and killed the assailants, before forming a mental link with the firestone – an alien artefact that contains part of the Vespiform brain and is responsible for assigning the creature’s mind. When the stone activated it linked with Lady Eddison’s mind and downloaded her knowledge of Agatha Christie. Thus when the Vespiform was given its conscience by the firestone it was endowed with the assumption that life on Earth resembled an Agatha Christie book – all of his actions that followed were inspired by the plots of her novels.
Reverend Golightly tries to claim the entire story is fiction but he is exposed when the Doctor goads him into anger – the emotion that triggers his metamorphosis. As he is overcome by spasms of buzzing the reverend explains that when his mind was born he vowed to take the fortune that was rightly his. His present thought turn to murder and he transforms into his true wasp-like form.
Realising her imagination is responsible for the creature’s behaviour, Agatha vows that it will also be the cause of it’s downfall. Clutching the firestone she runs from the house, the Vespiform and the Doctor and Donna close behind. Agatha climbs inside a car parked in the courtyard and drives away. The Doctor and Donna also commandeer a vehicle and they give chase, fearing that Agatha could be killed and as a result history will be changed forever.
The Doctor realises that the author is heading toward the lake, where she gets out of the car and steps toward the water. Having realised that her mind and the creature’s are linked, she resolves that if she kills herself then the Vespiform will die as well. However, Donna manages to save her life by snatching the firestone and throwing it into the nearby water. The Vespiform dives in after it and drowns as a result. The Doctor is unsettled by her murderous actions but she insists it was all she could do to save the day.
Suddenly Agatha doubles up in pain and the Doctor realises that if the Vespiform is dying then the mental link between them will cause Agatha to die too. However, before he can act a glowing light hits Agatha’s face and she passes out – still alive but unconscious. The Doctor explains that the Vespiform freed Agatha before it died; its last act was one of salvation.
He then tells Donna that Agatha will suffer from amnesia because of the near-fatal incident – she will not recall the Vespiform, the murders, or ever meeting either of the two travellers. This is the key to the mystery of Agatha’s disappearance – her car will be found by the lake the next day and she will arrive at a hotel a few days later with no memory of what has happened to her – the latter piece of the puzzle being fulfilled by the Doctor and Donna with the aid of the TARDIS.
The Doctor insists that the other guests at the party will carry on with their lives and Agatha will carry on with hers as well. Donna laments upon Agatha’s concern that her books will be remembered and the Doctor resolves to prove that they will. He hurries inside the ship and produces a large trunk from beneath the floor. Sorting through the various clutter inside – which include the chest plate of a Cyberman and an orb containing the trapped Carrionites – he produces a copy of one of Agatha’s books, “Death In The Clouds”, which bares on the cover a picture of a giant wasp. The two travellers realise Agatha’s memory might not be entirely missing from the encounter with the Vespiform then open the book and discover that the edition the Doctor has kept in his collection was printed in the year five billion – Agatha’s books really do last forever.