Poirot is thrilled to receive an invitation from renowned Belgian actress Marie Marvelle. She has been receiving anonymous notes about the Western Star, a valuable diamond purchased by her husband a cut-rate price several years before. The notes speak to the mystical nature of the diamonds and that they should be returned to their rightful owners. The next day, Lady Yardly claims to also have received similar notes about her own fabulous diamond, the Eastern Star. When Poirot and Hastings visit Lord and Lady Yardly the diamond is stolen in a daring robbery. Needless to say, none of this sits well with Poirot who finds he has a very tight knot to untie.