Poirot and Hastings are staying at a Cornish resort where Poirot believes someone is trying to kill the owner, Nick Buckley. When Nick's cousin Maggie arrives and is later found dead, Poirot launches an investigation. He discovers that Nick and a wealthy pilot named Michael Seton were secretly engaged, and Nick would inherit his fortune. Through various clues and schemes, Poirot eventually deduces that Nick herself was the killer, having engineered the death of her cousin Maggie to steal Maggie's identity and inheritance from Michael Seton.

1. Poirot and Hastings are staying at a Cornish resort. Conversing with Magdala "Nick" Buckley, Poirot
believes that someone's out to kill her, confirmed when he finds a bullet that Nick had thought to be a
wasp shooting past her head. Poirot explains his concern to Nick, who doesn't believe him at first.
Poirot suspects someone in Nick's inner circle: Nick's nearest living relative is a lawyer cousin, Charles
Vyse, who arranged the re-mortgaging on End House for her to supply desperately needed funds. Her
housekeeper's named Ellen, and the lodge near End House is leased by Australians Mr and Mrs Croft.
George Challenger has a soft spot for Nick. Nick's two closest friends are Freddie Rice, an abused
wife, and Jim Lazarus, an art dealer in love with Freddie. Nick was to undergo appendix surgery six
months earlier, and the Crofts suggested she make a will.
Poirot doesn't understand who wants Nick dead: Charles would inherit End House and Freddie would
get the rest of the estate – none of which is worth killing for. Poirot advises Nick to call a relative to
stay with her for a few weeks, and she calls her distant cousin Maggie. When Maggie arrives, Nick
hosts a party and everyone's present but George. Meanwhile, a renowned pilot named Michael Seton
has gone missing, sparking debate about his fate. Nick receives a call while the guests are enjoying the
party. Later, Maggie is found dead, wearing Nick's shawl. Nick and Maggie had gone to freshen up,
after which Maggie took Nick's shawl. George is relieved to see Nick alive. Realizing that Maggie was
killed by mistake under his nose, Poirot becomes furious, launching an investigation and finding Ellen
was working instead of enjoying the festivities, but she knows nothing. However, Ellen tells him
there's a secret compartment somewhere in the house, which Nick denies.
To protect Nick, Poirot tells everyone that she's going to a hospital and to leave her alone, and asks her
to not eat anything coming from an unknown place. The next day, the newspapers report that Michael
Seton is dead and Poirot correctly deduces that Nick received that information through the call. Nick
confesses to Poirot that she and Michael were secretly engaged. Michael was the sole inheritor of vast
wealth, and it's clear that wealth will go to Nick. Poirot suspects Freddie of trying to kill Nick to gain
the money, but also thinks that Charles may have tried to kill her, unaware of the will or its contents.
Poirot is wary of the Crofts: he asks Inspector Japp to inquire about them. Meanwhile, Poirot an d
Hastings find the love letters written by Michael, but don't find Nick's original will. Nick recalls
sending it to Charles. Poirot questions Charles, who denies receiving it. Mr Croft tells Poirot that he
sent the will to Charles; Poirot believes one of the men is lying.
Another attempt on Nick's life occurs when she eats a boxed chocolate, allegedly sent by Poirot
himself. The chocolates are laced with cocaine, but Nick is safe as she ate only one. The chocolates
were supplied by Freddie, who claims that Nick phoned her to bring the chocolates. Freddie found
Nick's voice a bit different. Poirot suspects Freddie, realizing that she's a cocaine addict with access to
its supply. Later, a note demanding money is found, without any names. Poirot finds some lov e letters
written to Michael and lies to everybody saying that Nick's dead, and convinces her to play dead.
Charles tells Poirot that he's received Nick's will, which is read in End House, awarding her money to
the Crofts for helping her father in Australia. Everyone's in disbelief, except the Crofts themselves;
Nick's "ghost" appears and it's revealed that the Crofts forged the will and sent it to Charles. Japp
reveals that the Crofts are forgers. When they heard Nick was dead, they sent a fraudulent will to
Charles. He arrests the duo, but Poirot claims that they had no hand in the murder. Just then, somebody
shoots at Freddie and misses. Poirot captures the man, Freddie's sick and dying husband.
Poirot reveals that the real murderer is Nick herself and that Michael was engaged to Maggie, not Nick.
Both have the same name, Magdala Buckley, but the correspondence had taken place between Michael
and Maggie. After learning of Michael's wealth and disappearance, Nick plotted to present herself as
her cousin to usurp his wealth. The plot was her own work, engineering a reason for bringing Maggie
to End House to kill her, preventing anybody from knowing the truth. George used to supply cocaine to
both Freddie and Nick. Nick used her supply to poison the chocolates . The note about money was from
Freddie's husband, who was constantly asking her for money. Nick is arrested, taking Freddie's cocaine
box as a souvenir. Poirot tells George to either surrender himself or go away, leaving Freddie (who's
stopped using cocaine) to recover from her addiction. The box taken by Nick contains a lethal dose of
cocaine, which she uses to escape the gallows. In the end, Jim and Freddie decide to marry.