Spots 
Mary Marvell receives ominous threat notes demanding the return of her famous diamond to an eastern deity. A parallel threat targets Lady Yardly and her matching stone, creating the illusion of a mystic curse. Poirot ignores the supernatural posturing to focus on personal rivalries and marital secrets. The mystery collapses when human vanity and financial desperation prove far more relevant than ancient prophecies.
I → II
An insurance company suspects Mr. Maltravers committed suicide to secure a payout for his young widow before going bankrupt. Dr. Bernard diagnoses a sudden internal hemorrhage, but Poirot digs deeper into the dead man's final hours. By putting guest Captain Black through a rapid word-association test, Poirot uncovers a story told at dinner about a farm suicide via rook rifle. The subconscious memory reveals how Maltravers learned to execute his fatal scheme.
III
Stolen naval blueprints and secret society vendettas lead Poirot to an unusually low-rent London apartment. International spy Elsa Hardt lures unsuspecting couples named Robinson into renting the flat as human decoys to absorb retaliatory strikes. Poirot ambushes the intruders in St. John's Wood and extracts the hidden stolen papers from inside a black velvet telephone cover. Meanwhile, an influenza-ridden Poirot must direct Hastings from bed when murder strikes a remote Derbyshire shooting box.
IV → V
In a high-value bank bond heist, suspicion centers on ocean transit, yet Poirot proves the stolen certificates were mailed on a faster ship before the boat even sailed. Later, a series of sudden deaths following the unsealing of King Men-her-Ra's tomb triggers widespread panic over an ancient Egyptian curse. Poirot travels to Cairo amidst desert heat to separate real poisons and psychological dread from archaeological legend. He protects young Sir Guy Willard by exposing human malice behind mystical fears.
VI → VII
Mrs. Opalsen's priceless pearl necklace vanishes from her locked hotel suite despite constant surveillance by her French maid, Célestine. The local police find a duplicate set of pearls hidden inside a mattress, but Poirot recognizes them as a planted distraction. By timing the physical movements required to open drawers and transfer items, he proves the maid lacked sufficient time. The heist collapses because the culprit valets left an undusted square mark on an empty room's table.
VIII
Prime Minister David MacAdam disappears in France during a wartime crisis, leaving police chasing false trails across the Channel. Poirot realizes the leader never crossed the sea at all, having been abducted between Windsor and London while an actor took his place behind bandages. Later, banker Mr. Davenheim vanishes from his country estate while his study safe is rifled. Poirot wins a wager with Inspector Japp by solving the disappearance entirely from his armchair using pure deduction.
IX → X
Eccentric benefactor Andrew Marsh leaves a will granting his educated niece Violet one year to locate his real fortune or forfeit everything to charity. Poirot travels to Crabtree Manor, discovering a hidden cavity behind a fireplace brick that contains only burned paper. Realizing the dead man anticipated standard search methods, Poirot re-examines the desk key envelope and holds it to a flame. Invisible ink reveals the authentic testament, proving that proper logic outwits elaborate mind games.
XI
Mastering deduction requires seeing beyond surface clutter to uncover the underlying human motives driving every crime.