The Adventure of the Cheap Flat (S02E07)

A stunning nightclub vocalist has gone rogue, absconding with classified US Navy submarine schematics, prompting FBI agent Burt's arrival in London to apprehend her. Burt regards Poirot as an ineffectual private investigator, while Poirot finds the FBI agent's arrogance off-putting. However, the urgency of the situation heightens when it's revealed that the plans could be seized by the Italian Fascists. Poirot's investigation takes an intriguing turn when he encounters a young couple thrilled about their incredible deal on a prime flat, recognizing a potential link to the espionage case.