Hastings was absent in the original story yet is present in the episode, while Miss Lemon is the reverse — she was present in the original story yet does not appear in the episode.
In the original story, Poirot finds out about the murder only after reading the newspaper. In the episode, Poirot was at the party where the chest containing Edward Clayton's corpse is found so he finds out after Japp calls on him to serve as a witness.
Downplayed. While the overall story is adapted from the novella of the same name, the names of the characters are taken from the short story "The Mystery of the Baghdad Chest" which was eventually remade into the novella "The Mystery of the Spanish Chest".
In addition, Lady Chatterton's first name was Alice in the short story and Abbie in the novella. Here in the episode, the adaptation takes a third option and names her Caroline.
Asshole Victim: Edward Clayton is a cold man who emotionally abuses his wife. When Lady Chatterton calls on Poirot for assistance, it's to help prevent Edward from potentially killing his wife, Marguerite, given his suspicious actions towards her.
Bait-and-Switch: The episode starts with Lady Chatterton calling on Poirot to help protect her friend Marguerite Clayton from her abusive husband, hinting her to be the victim. As it turns out, it's Edward who then gets murdered.
Big Damn Heroes: Major Rich saves Poirot's hide from an enraged Colonel Curtiss at the climax of the episode.
Book Ends: The episode starts with a flashback to when Colonel Curtiss demanded a fencing duel against a fellow officer who made a joke at Mrs. Clayton's expense. The episode's climax occurs at the very same gymnasium, where Curtiss then duels Major Rich after the latter saves Poirot. The latter duel is shown interspersed with flashbacks to the former.
Dude Magnet: Marguerite Clayton is pursued by no fewer than three men, her husband included. This leads to deadly results.
Eye Scream: Implied. Edward Clayton made the hole in the chest so that he could breathe and see if his wife really was cheating on him. The murderer then stabs through said eye hole, killing Clayton.
Good Scars, Evil Scars: Colonel Curtiss has a nasty one on his cheek gained from an honor duel as shown in the prologue.
I Take Offense to That Last One: After Colonel Curtiss delivers his Motive Rant to Poirot, Poirot starts rebutting him by insisting he's not a "bloody little frog", but a "bloody little Belgian". Poirot's annoyance at being mistaken for a Frenchman strikes again.
Monochrome Past: The duel at the beginning is shown in monochrome/sepia tones.
Murder the Hypotenuse: Exaggerated. Colonel Curtiss planned the whole affair to get Edward Clayton out of the way by killing him, and by ensuring his other rival, Major Rich, is framed for the matter by murdering Clayton at Rich's residence, by implying that Marguerite was cheating on Edward with Major Rich and having him hide in the titular chest so he could confirm it, unaware that Curtiss would use said chest as his proverbial coffin.
Officer and a Gentleman: Colonel Curtiss provides a deconstruction. As a military officer honor, especially the honor of a woman, would be important to him; however, that combined with Curtiss' feelings of entitlement towards Marguerite Clayton leads him to murder one man and frame a fellow officer to get them both out of the way.
Politically Incorrect Villain: Colonel Curtiss is shown to be a rather racist man. He decries jazz music as "second-rate music provided by racial inferiors", and later calls Poirot a "bloody little frog", pointedly ignoring his Belgian origin while using a slur meant for French people.
Sword Cane: Colonel Curtiss, an avid duelist, carries one around. It's even shown to be the murder weapon.
Surprisingly Realistic Outcome: Poirot confronts Colonel Curtiss alone, with no backup. Unfortunately for Poirot, the colonel is an avid duelist and didn't come unprepared, so Poirot is left doing The Summation at swordpoint and would have likely been killed had it not been for Major Rich's timely arrival.
