The Jewel Robbery at the Grand Metropolitan

While staying at the Grand Metropolitan Hotel in Brighton, Poirot and Hastings meet the wealthy Opalsen couple. Mrs Opalsen offers to show Poirot a pearl necklace, but discovers that it has been stolen when she goes to fetch it from her room.

Suspicion falls on Mrs Opalsen's personal maid, Célestine, and the hotel chambermaid assigned to clean the room. Célestine has orders to be present in the room whenever the chambermaid is there, but admits that she twice stepped out briefly to fetch items from her own adjoining room. Each woman blames the other for the theft, and the police search them but find no trace of the necklace on either one. Poirot determines that during the time it took Célestine to leave and return, the thief could not have found Mrs Opalsen's jewelry box, unlocked it, taken the necklace, relocked the box, and returned it to its drawer. A necklace is found hidden in Célestine's bed, but Poirot spots it as a fake. He and Hastings examine an adjoining empty room and question a hotel valet who looks after Mr Opalsen's room, directly across the corridor from his wife's. Poirot presents a card for both the chambermaid and valet to examine, but neither admits to having seen it before.

Poirot abruptly returns to London, then reports to Hastings and the Opalsens the following evening that the case has been solved, the necklace recovered, and the thieves arrested. The chambermaid and valet had conspired to steal the necklace. When Célestine left the room, the chambermaid took the box from its drawer, which had been lubricated to minimise noise, and passed it to the valet waiting in the empty adjoining room. He unlocked the box, took the necklace, and later passed the box back to the chambermaid so she could return it to the drawer once Célestine left again. The card that Poirot had them handle was treated to yield good fingerprints, which he gave to Japp so the latter could confirm the pair's identities as known jewel thieves and make the arrest at the hotel.