Poirot visits a Cornish seaside resort and meets Nick, a young girl attached to a crumbling and dilapidated house by the seaside with a mortgage. So when she informs Poirot that she had several near escapes, his detective streak comes into play. There are a few potential suspects but it seems unlikely any of them would profit from the paltry inheritance Nick would leave in her will after her death. The suspects are Freddie Rice, a habitual cocaine user; Commander Challenger, who seems to be in love with Nick; Jim Lazarus, an art dealer in love with Freddie and also Freddie’s husband who refuses to grant her a divorce. Then there is also Charles Wyse, Nick’s appointed solicitor. In addition, there are two lodgers using the garden cottage at End House, Mr. and Mrs. Croft, who say they are Australians.
So Poirot suggests Nick call her cousin, Maggie Buckley, for protection. But an unfortunate incident involving the exchanging of coats leads to Maggie’s death because she was wearing a black dress. This baffles Poirot until he assumes the murder was due to the coat exchange. When he goes to interview Nick – whose true name is Magdala – she says to his puzzlement after a telephone call, that she has nothing to live for left. He then puts two and two together to figure out she had been secretly engaged to Arthur Streeton, a pilot who has been missing for some time and she had received news of his death. He sends her to a nursing home for protection while he investigates End House.
The Crofts arouse his suspicion and so does the love letters written to Magdala. But he is then informed that Nick has almost died of poisoning by chocolates, purported to have been sent by him. He calls in Miss Lemon for assistance. Using the help of Nick to stage a séance to talk with the dead through a medium during an arranged early reading of the will, something very odd comes to light. Her will leaves her inheritance to an unexpected party. But the fun doesn’t end there. Hercule Poirot reveals a charade has been going on under his nose the whole time and points out the true murderer of the Magdala Buckley that was engaged to Arthur Streeton who had a considerable fortune left to him by his uncle. He says he was inspired by the conversation on nicknames between Miss Lemon and Captain Hastings to reverse his original thought process.
The TV episode of Poirot doesn’t change the plot much because there was only one major change: the attempted assassination in front of Poirot did not happen in a lonely garden in the back of the hotel but a crowded spot near the swimming pool.