NOTE: This spoiler was submitted by Jeremy

1914, Belgium – During World War I, Allied forces are devising a plan of attack at the Yser Bridge. Captain Rens (Orlando Seale) plans to wait for their attack, but Hercule Poirot (Kenneth Branagh) suggests an alternative – they attack within seven minutes while obscured by a wave of gas to prevent the enemies from seeing them. The soldiers go along with Poirot’s plan and launch a successful attack against the Central Powers. Just as Rens congratulates Poirot for his plan, he accidentally trips a bomb that blows up the bridge and kills Rens, plus wounds several soldiers and Poirot himself.

Poirot wakes up in the hospital by his nurse, Katherine (Susannah Fielding), who is also his lover. He tells her she shouldn’t have come, but she insists that she loves him. He turns around to reveal his disfigurement from the explosion, to which Katherine simply suggests that he grow a mustache.

1937, London – Poirot attends a nightclub where jazz singer Salome Otterbourne (Sophie Okonedo) is performing. Her niece, Rosalie (Letitia Wright), handles the snooty maitre ‘d and gets her payment. On the dance floor are a young couple, Simon Doyle (Armie Hammer) and Jacqueline “Jackie” de Bellefort (Emma Mackey). They dance and kiss passionately as Salome sings. The club is then visited by Jackie’s friend, wealthy heiress Linnet Ridgeway (Gal Gadot). She catches up with Jackie, asking her if she can offer Simon a job before they get married. After Jackie introduces the two of them, Simon and Linnet share a dance, one with equal, if not greater, passion than the one Simon shared with Jackie, and she looks on with discomfort.

Six weeks later, Poirot is in Egypt. He sits by the Sphinx to enjoy tea and a cake when he notices a man flying a kite on the pyramid. He goes to admonish the man until he sees it is his old friend Bouc (Tom Bateman). He is elated to see Poirot and brings him to meet his mother, Euphemia (Annette Bening). She is unimpressed with meeting Poirot and focuses more on her painting of the pyramid.

Bouc brings Poirot to the First Cataract Hotel where they attend a party in honor of the newlyweds – Simon and Linnet. Among the guests are Euphemia; Salome and Rosalie; Linnet’s maid, Louise Bourget (Rose Leslie); Linnet’s godmother Marie Van Schulyer (Jennifer Saunders) and her nurse Mrs. Bowers (Dawn French); Linnet’s “Cousin” Andrew Katchadourian (Ali Fazal), who oversees her will; and Dr. Linus Windlesham (Russell Brand), Linnet’s former lover. Bouc tells Poirot how Marie leans toward communism and hates how wealthy Linnet is, but Mrs. Bowers covets her wealth, as does Louise, while Andrew is a “slippery fish” that nobody but Linnet trusts. The celebration is brought down by Jackie’s surprise appearance, which is an occurrence that has been going on since Simon left her for Linnet.

While touring the area, Linnet and Simon approach Poirot and ask him to investigate Jackie since her constant stalking of them has made Linnet worried and uncomfortable. Poirot finds Jackie and speaks to her, learning that Linnet has always cast a shadow over her. Jackie had been cast in the school play of “Antony and Cleopatra”, but their teacher replaced her with Linnet. Jackie believes Simon still loves her, and she is also carrying a .22 caliber gun in her bag. Poirot brings this information to Linnet and Simon, suggesting they go home and start their lives, but Simon opts for a different solution.

The next day, Simon invites the guests onto his yacht, the SS Karnak, to continue enjoying the festivities. Linnet later finds Poirot in his room and tells him that she does not trust any of the guests. Poirot later sees as Andrew approaches Linnet to sign for an acquisition of property, which makes Mrs. Bowers unhappy. Rosalie chimes in, which makes Andrew back off.

Later that night, while the guests are partying, Poirot and Bouc chat, with Bouc telling Poirot that he is in a relationship with Rosalie and is very much in love with her, but he is hoping for his mother’s blessing. He also notes Poirot’s attraction to Salome and encourages him to find love.

The party later stops at the Abu Simbel. While there, Bouc tries to tell Euphemia about his relationship with Rosalie, though she expresses her disapproval. Regardless, he is glad that he no longer feels the need to hide their romance. Poirot also talks to Salome, who tells him he wears a mask. Not long after, Simon and Linnet are together when they are nearly crushed by a falling boulder. Moments later, a sandstorm forces the guests to take cover. After everything settles, they board the boat again to find that Jackie has made it onboard. Simon says they cannot do anything since she has a paid ticket to board.

The guests have dinner, with Simon and Linnet planning on leaving the following morning. Poirot has some champagne and feels nauseous. He goes outside and finds Jackie, with the two of them talking about their failed loves. Poirot mentions that Katherine was killed in an explosion, and the loss broke him into what he is now. After Poirot goes to bed, Jackie finds Simon and Linnet, the two taunting her over how many times they made love that day. After Linnet goes to bed, Simon and Jackie argue, leading her to pulling out her gun and shooting Simon in the leg. She becomes mortified at what she has done and throws the gun to the side. Mrs. Bowers tends to Jackie as she gets hysterical, while Windlesham tends to Simon’s leg.

The following morning, Louise enters the couple’s room and screams in horror. Linnet is dead, with a bullet hole in her head. It is said that she was murdered in her sleep. Additionally, a valuable jewel necklace that Linnet was wearing has been stolen. Although Jackie is the prime suspect, Mrs. Bowers insists that she never left the bed overnight. Simon sees his wife’s body and wails in agony.

After Linnet’s body is removed, Poirot has help from Simon and Bouc to interrogate the guests. They start with Louise, who tells the men that she was previously engaged and set to leave Linnet’s employment after her marriage, but Linnet caused Louise’s fiance to break it off after she offered to pay off his debts in a test of his affection for her. Next, Poirot and Bouc speak to Windlesham, who admits that he still carried a torch for Linnet and contemplated suicide after learning of her engagement. He was originally “Lord Windlesham” but wished to pursue a career in helping others. The men then talk to Andrew, who reveals that the contracts he intended for would have cost Linnet her money and estate. Andrew is also revealed to be carrying a .45 caliber gun. Bouc then gets Marie and Mrs. Bowers in for interrogation. It is revealed that Mrs. Bowers came from a wealthy family, but Linnet’s family forced them off their land during the Great Depression. Poirot also deduces that Marie and Mrs. Bowers are lovers.

Shortly after, Jackie’s gun is fished out of the water, wrapped in Marie’s missing scarf and shown to have a bullet hole in it. The gun is missing two bullets, not counting the one Jackie shot Simon with. Poirot speaks to Salome, who reveals that she did not previously get along with Linnet due to her having once complained about being in a pool with Salome (a woman of color) as a result of an upbringing by Linnet’s terrible father, but later on, Rosalie and Linnet got along quite well. Salome also happens to be carrying a gun like the one Jackie had. Euphemia interrupts by announcing that she found Linnet’s necklace in her room.

After investigating Euphemia’s room, a confrontation occurs where Poirot reveals that Euphemia hired him to spy on Rosalie to see if she was really good for Bouc. Poirot says Rosalie is a good person for Bouc, but Rosalie is angered by Poirot’s revelation and berates him. He follows her, but the two soon discover Louise’s body in the water wheel. Her throat had been cut by a scalpel missing from Windlesham’s collection of knives. Money is also found on her person. Poirot notices blood splatter on the wall, slightly covered by a potential witness to Louise’s murder.

The guests lock themselves in their rooms for safety. Poirot conducts one more interview with Simon, and he begins to interrogate Bouc, who is stunned that his friend is accusing him. He figures that Bouc had found Linnet dead, and that he was the one who stole her necklace and planted it in Euphemia’s room to have her framed so that he and Rosalie can be free from her. Bouc had also witnessed Louise trying to bribe the killer before she was murdered, and Bouc threw his coat into the river since it was covered in her blood. Before he can reveal who he saw, a gunshot rings out and hits Bouc in the throat, killing him. Poirot chases after the killer but fails to catch them.

Poirot then has the guests locked in the bar with the help of the crew, as he is ready to put the killer in their place. Poirot knows that Andrew was the one who pushed the boulder that almost killed Simon and Linnet, which he admits was an act of desperation, but he says he did not kill anybody. It is revealed that Linnet’s killer was Simon, and that he was working with Jackie, who is the mastermind behind everything. They planned for Simon to marry Linnet and take her fortune after her death. Jackie shot Simon with a blank, and after he shot Linnet, he shot himself for real and used Marie’s scarf to silence the gunshot. He also stole red paint from Euphemia to fake the injury before actually shooting himself. Simon got rid of the gun, but because Louise was a witness, Jackie killed her, and then killed Bouc because Simon gave her a signal just before he could reveal Jackie’s identity. The two lovers realize they are caught and aim a gun at the guests. Seeing no other options, Jackie kisses Simon one last time before she shoots him in the back, the bullet hitting her too. They die in each other’s arms.

The guests leave the boat. Windlesham says he will continue his work in Africa. Euphemia and Rosalie still dislike Poirot but are thankful for finding Bouc’s killer. Salome also admits to Poirot that she had interest in him.

Six months later in London, Salome is performing at the same nightclub from earlier. Poirot attends, showing that he shaved his mustache to reveal his scars. He watches Salome perform.

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Hercule Poirot joins a wedding party with his old friend Bouc in Egypt. The newlyweds are Simon Doyle and wealthy heiress Linnet Ridgeway. Complicating things is Simon's scorned former fiance, Jackie de Bellefort, who shows up and intimidates Linnet. Among the other guests are Bouc's mother Euphemia, jazz singer Salome Otterbourne and her niece Rosalie (who is also in a relationship with Bouc), Linnet's godmother Marie Van Schulyer and her nurse/lover Mrs. Bowers, Linnet's maid Louise Bourget, Linnet's family friend "Cousin" Andrew Katchadourian, and Linnet's former lover, Dr. Linus Windlesham.

After escaping death by a falling boulder during a stop on their tour of the Nile, Linnet is later found murdered by a gunshot to the head. Jackie is the prime suspect because the murder weapon was her own .22 caliber gun, but Mrs. Bowers had been tending to her overnight. As Poirot interrogates the other passengers, Louise is found murdered as well. Poirot interrogates Bouc since he had stolen Linnet's jewel necklace and planted it in his mother's room to frame her so he can be with Rosalie in peace. Before Bouc can reveal who he had seen that killed Louise, he is murdered as well.

Poirot locks the guests in the bar as he comes to deduce that the killers are none other than Simon and Jackie. Jackie was the mastermind, having planned from the beginning for Simon to marry Linnet and inherit her wealth after her death. Simon shot Linnet with Jackie's gun, and Jackie killed Louise since she witnessed the murder and tried to extort her, then later killed Bouc before she could reveal her identity. Jackie kills herself and Simon by shooting him through the back so the bullet would hit her too.

Poirot later appears to try and pursue a relationship with Salome as he watches her perform in London, and he shaves his trademark mustache as a means to let his guard down.