Hercule Poirot is hired by Mary Marvell, a famous movie actress, because she has received strange anonymous messages demanding the return of her magnificent diamond, the "Star of the West," before the next full moon. But that's not all: Lady Yardly, a powerful society lady who owns a twin diamond, the "Star of the East," has received similar threats. Can Poirot protect the valuable jewels? And what will he do if they disappear?