Since Stillingfleet is Adapted Out, the role of taking in Norma during the story is taken up by Poirot himself instead.
The method through which the murderer gaslights Norma into thinking she committed the murder is changed; instead of using the door numbers to manipulate Norma into thinking she killed the victim, she's instead misled through the use of two identical knives, one in her drawer and another at the murder scene, that would make her think she was the murderer.
The portrait of Andrew Restarick is commissioned and unveiled openly for all to see. In the novel it was painted by David in secret, and he was killed to hide the fact that it was made recently.
Unlike her more neutral counterpart from the novel, Claudia is so openly hostile towards Norma that Poirot brings it up as a possible motive near the end. This is implied to be at least in part due to Andrew/Robert poisoning Claudia's opinion of her.
The real Andrew Restarick is revealed to have despised his daughter Norma and abandoned his lover and other daughter.
Adaptational Villainy: Frances Carey is not just Robert Orwell accomplice in this version. As Andrew Restarick's other daughter she has an equal stake in stealing Norma's inheritence, and the cruelty of her plan turns out to be born out of deep resentment of her legitimate half-sister.
Dr Stillingfleet is nowhere to be found, and his role as Norma's love interest is given to David Baker.
Miss Lemon was present in the original story, but is not in this episode.
The subplot about Sonia being a spy feeding information to the Herzegovinian Embassy is removed.
Bastard Bastard: Frances Cary in the adaptation is revealed to be the daughter of Andrew Restarick by Norma's tutor A.J. Battersby, and plots to inherit her half-sister's fortune by getting her killed, either by execution or more direct means.
Bath Suicide: It is revealed that Norma Restarick's mother Mary, who was distraught by the absence of her husband Andrew, committed suicide by slitting her wrists in the bathtub, traumatizing Norma for so many years. Through the same years, her half-sister Frances exploited her trauma and used it to prey upon Norma through actions, including the ice cream and the murder of Nanny Seagram. With Poirot's help, however, Norma is able to get over the trauma and later use the trope as a reenactment by Faking the Dead in order to prevent Frances from killing her should her "father" be exposed as an impostor.
Break the Cutie: Norma is forced to relive the trauma of her mother's Bath Suicide over and over again until she is on the brink of insanity.
The role of Louise Charpentier as the first murder victim is taken up by Nanny Seagram as the only murder victim in the adaptation.
The character of A.J. Battersby, Norma's tutor and later schoolmistress, is added to the story, taking up elements of the book version of Mary Restarick in the process.
Norma dates David Baker in the novel, but ends up with Dr Stillingfleet after he plays a part in saving her life. Since Stillingfleet is omitted, she ends up with David here, who is alive and has a more heroic role.
Norma had a deceased mother and a stepmother named Mary in the novel. Here she has no stepmother, and Mary is the name of her birth mother.
Decomposite Character: Norma's stepmother Mary from the novel is turned into her deceased birth mother in the series. While this technically makes her a Composite Character as well, the novel reveals at the end that "Mary" doesn't exist and is just Frances Cary in disguise. Frances and Mary are separate people in the series.
Dies Differently in Adaptation: Louise Charpentier in the book was killed by being pushed out of her apartment window. Her show counterpart, Nanny Seagram, is smothered by a pillow instead, and had her wrists slashed post-mortem to incriminate Norma.
Dies Wide Open: Nanny Seagram is found dead with a look of absolute terror on her face.
Disappeared Dad: Andrew Restarick abandoned his daughter Norma and her mother many years before the events of the novel and went to Africa to seek his fortune. By the events of the story he has returned to try and make amends with his daughter. Except he didn't, since Poirot realises that he disappeared so efficiently, no one actually knew what he looked like — and by extension, whether the man calling himself Andrew Restarick actually was him...
Faking the Dead: Near the end, Norma appears to have killed herself in the bathtub by slitting her wrists in the same manner as her mother. However, it turns out that Norma has heeded Poirot's helpful advice by using the memory of her mother's suicide as a reenactment and faking her own in order to prevent her half-sister from smothering her with the pillow that was used to kill Nanny Seagram in case her "father" would be exposed as an impostor.
Ariadne Oliver, unimpressed by Frances Carey's tea brewing-skills, offhandedly asks "Where was the girl brought up?". The answer to that question turns out to be crucial to the case at hand.
Ariadne asks Poirot how he knew that David was genuinely in love with Norma all along. Poirot points out that Ariadne told him herself — David was looking at Norma throughout the entire party, even as Frances was trying to seduce him.
Gold Digger: Everyone suspects that Sir Roderick's secretary Sonia is trying to seduce him because he potentially stands to inherit Norma Restarick's fortune if anything happens to her. She turns out to be genuinely in love with him.
Meaningful Echo: "Can you save me?" First said by Norma, but it also turns out to have been her mother's last words before she died.
Men Don't Cry: At the end, when Poirot sees that Norma is finally able to smile in freedom, he starts shedding a Single Tear. Ariadne Oliver asks if he is crying, but Poirot says that it's "only the breeze".
Not So Stoic: Poirot is uncharacteristically furious at the culprits who tried to Gaslight Norma.
Not What It Looks Like: Norma walks in on David painting Frances in the nude and assumes things went further. While it turns out that David only has eyes for Norma, Frances was deliberately throwing herself at David as part of her plot to destroy Norma's life.
Obfuscating Stupidity: Sir Roderick initially seems to be a hapless old "Colonel Blimp" type who gets suckered into Poirot's ruse about being an old friend of his, but is quickly revealed to be a lot sharper than he first appears.
Related in the Adaptation: Frances Cary becomes a half-sister of Norma Restarick, whose father left her mother for her teacher, Miss Battersby, with whom he had a relationship.
Rule of Symbolism: Norma slashes up the new portrait of Andrew Restarick, which is ultimately explained to be because it was a portrait of an impostor.
Setting Update: Especially noticable in this episode: the original novel is set in and heavily reflects the culture of The '60s and comes off as an Unintentional Period Piece, but is transported back thirty years in the adaptation.
Shout-Out: Concierge Alf Renny, Ariadne Oliver's "#1 fan", has read her novel called Lady, Don't Fall Backwardsfour times and still has no idea who committed the murder. This is a shout-out to an episode of the TV series Hancock's Half Hour ("The Missing Page"), in which Tony Hancock tries to find out who committed the murder in a book he'd just read with a missing page.
Shrine to the Fallen: It is said that Norma has turned her bedroom into a shrine for her mother, who committed suicide when Norma was seven.
Spared by the Adaptation: David Baker, mostly by being melded with the character who is Norma's love interest in the original book.
The Thing That Would Not Leave: Mrs Oliver invites herself over for tea at the girls' apartment, where she ignores their increasingly obvious hints that they want her to leave.
Trauma Button: Norma feels intense guilt over convincing her nanny Seagram to stop for ice cream the day her mother died, feeling she could have been saved if they arrived earlier. Frances is revealed to have weaponized this by serving ice cream at their party. The murder of Seagram was also staged to mimic the suicide of Norma's mother, which was also meant to reawaken her trauma.
Vorpal Pillow: It is revealed that Frances Cary, Norma Restarick's half-sister, murdered Nanny Lavinia Seagram for threatening to reveal the identity of Robert Orwell as the impostor of Norma's father Andrew by using the pillow as a weapon. Afterward, Frances took her half-sister's knife from her bedroom and slit Seagram's wrists with it in order to force Norma to relive the trauma of her mother's suicide and make her believe she did it. Later, when Norma is finally able to reveal Robert's deception, Frances attempts to smother her half-sister herself with the same pillow, but Norma cleverly fakes her own death using the memory of her mother's bathtub suicide as an idea to make her half-sister believe she killed herself in the same manner as her mother.
When She Smiles: It's frequently discussed how Norma Restarick has hardly ever smiled since the death of her mother... except whenever she sees David Baker, at which point she simply lights up. At the end when Norma hugs David again, she makes an Aside Glance in front of Poirot and is finally able to smile again, moving him to the point of tears.
Unintentional Period Piece, but is transported back thirty years in the adaptation.