There is a Tide … (1948) – Agatha Christie | First Edition Identification Guide

In spring 1946, Lynn Marchmont, a Wren during the war, settles again in her mother’s home in the village of Warmsley Vale in peacetime, at first happy to be home, then finding life dull. She has been engaged to farmer Rowley Cloade since before the war.
David guards his sister and her fortune. Mrs Marchmont gets up the courage to ask Rosaleen for five hundred pounds when David is away, and Rosaleen writes a check. David angrily turns down a request from Frances Cloade, whose family is in desperate need of money.
A man calling himself Enoch Arden arrives at the village inn The Stag, and attempts to blackmail David by saying he knows how to find Rosaleen’s first husband, Robert Underhay. Their conversation is overheard by the landlady, who tells Rowley Cloade. A few days later, a maid finds Arden’s body in his room with his head smashed in. That same day, David was down from London. He met Lynn on his dash to catch the last train to London, and evidently telephoned her from the London flat shortly after 11 pm, arguing first that he loves her and then that he cannot be good for her. As the murder is believed to have happened before 9 pm, he had enough opportunity and motive and he is arrested.
Rowley Cloade appeals to the detective Hercule Poirot to find the true identity of the dead man. Poirot asks Major Porter. Rosaleen sees the body and says she does not know the man at all. At the inquest, Porter says that Arden was indeed Robert Underhay, and the coroner’s jury votes against advice to believe him, voting wilful murder by David. The estate will revert to the Cloades, if the jury’s decision holds, as it means Rosaleen’s second marriage was not valid.
Poirot talks to people in the village, including Mrs Leadbetter, a guest at the inn who saw a ‘hussy’, a heavily made-up woman wearing slacks and an orange scarf, enter the room assigned to the dead man after 10 pm on the fateful evening. As the police believe that David was on the train to London at that time, they let him go, and look more seriously at women. Poirot learns how the death occurred, with a fall on the marble fender of the fireplace. He proposes it was accidental death, not intentional murder.
Lynn is falling in love with David. Major Porter commits suicide in London but leaves no note. Poirot realises from a photo at the home of Frances Cloade that Arden was related to her; she admits Arden was Charles Trenton, her second cousin. She came up with the plan to blackmail Rosaleen after hearing Major Porter’s story about Underhay. This explains Arden’s true identity, but nothing else. Frances denies bribing Major Porter.
When Poirot and Lynn visit Rosaleen at home, the maid realises she does not respond. Poirot sees that Rosaleen has died in her sleep. The doctor identifies the harmless sleeping powder she took, at her bedside table. Superintendent Spence, the investigating officer, suggests that she was the murderer. Rosaleen died from too much morphine; where did she get it?
Lynn tells Rowley that she wishes to marry David Hunter and he explodes in anger. Rowley is strangling Lynn when Poirot enters the house, and Rowley stops. David arrives shortly, and Poirot explains everything. Rowley visited Arden, and seeing the resemblance to Frances, reacted angrily to the deception. Punched by Rowley, Arden fell against the marble fireplace fender and died. Rowley saw the opportunity to incriminate David. He smashed in Arden’s head with fire tongs and left David’s lighter at the scene. Rowley persuaded Porter to give the false identification by offering him money. Then Rowley employed Poirot, who would be sure to go to Porter. Porter’s guilt led him to commit suicide, leaving a note that Rowley destroyed. David was about to pay the blackmail to Arden; upon discovering Arden’s body, David ran for the late train but missed it. David then backtracked to The Stag, disguised himself as a woman, and played out the scene, heard by Mrs Leadbetter, which established the later time of death. Then he called Rosaleen, who placed a call to Lynn that was delivered by the operator but then cut off. A minute later, David called Lynn from the local train station, giving her the impression that he was calling from London. He returned to London on the early morning milk train the next day.
Of the three deaths, Poirot says one is accidental, one is a suicide, and one is murder. The true murder victim was Rosaleen. The woman known as Rosaleen was not David Hunter’s sister; his sister was killed and the housemaid Eileen Corrigan had survived the bombing. Poirot sent to Ireland for a photo of Eileen, which explained the truth of what happened during the war. David persuaded her to be Rosaleen. Now he could kill this accomplice with a morphine pill and marry Lynn, whom he loved. Poirot knows all these details but only some of them are known to the police. Rowley feels guilty about the deaths of Trenton and Porter. The police focus on David killing Rosaleen/Eileen, and proving all of that case. After a while, Lynn returns to Rowley, realising she does love him, especially after his show of force. Life will not be dull with him.