Plot

Two wealthy women, Marie Marvelle and Yardley Chase, receive threatening letters.

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 at 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 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.—garykmcd

Poirot is excited when Belgium's greatest film star asks him to meet her while she is in London. But when he arrives he finds her terrified by a series of threatening notes hand-delivered to the hotel's front desk from a Chinese man, stating that a precious diamond she owns will be stolen. The next day, Poirot receives a call from another woman who owns a matching diamond, stating that she too has received threatening notes, but this time through the post. When both diamonds disappear, it leads Poirot to cross paths with the adulterous, thieving husband of one of the woman, and a rather shady German businessman who likes to collect diamonds for his private collection -- and he doesn't care how the person who has them comes into possession of the diamonds.—Spirit11

Poirot receives a visit from Miss Mary Marvell (Rosalind Bennett), the famous Belgian film star on her visit to London. She has received three letters, handed to her by a Chinese man, which warn her to return her fabulous diamond jewel, the "Western Star", to where it came from - the left eye of an idol - before the next full moon. Her husband, Gregorie Rolf (Oliver Cotton), who bought it from a Chinese man in San Francisco, gave Mary the jewel three years ago. The pair are going to stay at Yardly Chase, the home of Lord (Alister Cameron) and Lady Yardly (Caroline Goodall) when the moon is next full to discuss the making of a film there and Mary is determined to go with her diamond. Both Poirot and Hastings remember society gossip from three years back that linked Rolf and Lady Yardly.

The Yardlys also own an identical diamond that came from the right eye of the idol - the Star of the East. After Mary has gone Poirot goes out and Hastings receives a visit from Lady Yardly, who was advised to visit Poirot by her friend Mary Cavendish. Hastings deduces that she too has received warning letters. Her husband plans to sell their jewel as he is in debt. When Poirot learns this, he arranges to visit Yardly Chase and is there when the lights go out and Lady Yardly is attacked by a Chinese man and her jewel stolen. The next day, Mary's jewel is stolen from her London hotel.

Poirot tells Hastings that there never were two jewels or any Chinese man - it was all an invention by Rolf. Three years before in the USA he had an affair with Lady Yardly and blackmailed her into giving him the diamond which he then gave to his wife as a wedding present. Lady Yardly's diamond was a paste copy that would have been discovered when her husband sold it.

Lady Yardly was starting to fight back against her blackmailer and Rolf arranged the deception against his wife to steal the diamond & sell it in the black market. Lady Yardly copied this deception when Hastings told her of the threats (Lady Yardley underwent the whole charade of a Chinaman stealing her diamond, when she herself had just take the diamond & rolled it down the corridor & appeared to have been injured by a Chinese burglar).

Poirot's threats manage to persuade Rolf to give the real diamond back and leave the Yardlys in peace. Rolf agrees and promises to return with the diamond. Poirot knows that Rolf will try to sell the diamond. Poirot contacts the diamond merchant who helped Rolf forge the Diamond 3 years back, & to whom Rolf is trying to sell the diamond now. The diamond merchant refuses to buy the diamond from Rolf & instead switches the fake one with the real one that Rolf had. The diamond merchant returns the real jewel to Poirot. This forces Rolf to contact another buyer & hastily arrange a meeting before he must leave the country. But the buyer recognizes the fake for what it is & refuses to buy it, leaving Rolf with no money.

Poirot returns the Yardly's real jewel to them. Poirot communicates the truth to Mary & tells her how her husband acquired the diamond for her in the first place. This closes any chance Rolf had of returning to his wife.