Chapter X of “The Mysterious Affair at Styles” begins with the narrator, Hastings, expressing frustration over not being able to find Poirot, only to learn he might have gone to London. As Hastings ponders the situation, especially the impact of Dr. Bauerstein’s arrest on Mary Cavendish, he decides to share the news with John, who is astonished but agrees to keep it discreet for the time being. The expected public announcement of the arrest doesn’t appear in the newspapers, leading to speculation about further arrests and Poirot’s strange absence and activities.

Upon Poirot’s return from London, he feigns ignorance about Bauerstein’s arrest, leading to a confusing conversation where Poirot reveals Bauerstein is actually arrested for espionage, not the murder of Mrs. Inglethorp, contrary to Hastings’s understanding. Poirot highlights Bauerstein’s suspicious behavior and foreign origins, painting him more as a patriot to his country than a criminal, while subtly hinting at his own suspicions about the case.

Hastings and Poirot discuss the case further, with Poirot receiving a mysterious clue from Miss Howard and elaborating on his methodical approach to solving the case relying on his “little grey cells.” Poirot expresses concern over making a critical decision that could affect a woman’s happiness, indicating a deeper, personal involvement in the outcome beyond simply solving the mystery.

Mary Cavendish’s interaction with Hastings reveals her unhappiness in her marriage to John and her independent spirit, longing for freedom. Her dismissive reaction to Bauerstein’s arrest contrasts with Hastings’s suspicions about her possible feelings for the spy.

The chapter also presents Poirot’s cunning in obtaining a crucial piece of evidence without alerting the hospital staff, leading to the discovery of Lawrence Cavendish’s fingerprints on a bottle of strychnine. This finding deepens the mystery and complicates the investigation, as strychnine has been a recurring element in the case.

Poirot and Hastings’s interactions with Mary Cavendish and the village’s subtle dynamics hint at underlying tensions and secrets, driving the narrative towards unraveling the complexity of Mrs. Inglethorp’s murder against a backdrop of espionage and personal turmoil within the Styles household.