"The Adventure of the Western Star" is from the second season of Poirot, and it concerns a flawless diamond that represents one eye of a Chinese god. There is another one, the diamond representing the other eye, that is known as the "Eastern Star." Poirot is thrilled to be called to the hotel of a great Belgian film star. It turns out that she owns one of the diamonds. She has received threatening letters delivered to the hotel by a Chinaman, warning her that her diamond should be returned to its rightful owners and will be stolen.
The next day, an English woman who owns the other diamond calls and reports the same threats. When Poirot and Hastings visit her and her husband, the diamond is stolen nearly in front of them, and a piece of the Chinaman's robe is found caught in the doorway.
This story is extremely well done, though as a mystery, it's not Christie's best. In fact, I found it fairly obvious. But it is enjoyable, and after seeing the morose Poirot in Murder on the Orient Express, it was refreshing to see this earlier episode.