A banker, Mr. Alexander Holder of Streatham makes a loan of 50,000 Pounds to a socially prominent client, who leaves the Beryl Coronet - one of the most valuable public possessions in existence - as security. Holder feels that he must not leave this rare and precious piece of jewellery in his personal safe at the bank, and so he takes it home with him to lock it up there. He is awoken in the night by a noise, enters his dressing room, and is horrified to see his son Arthur with the coronet in his hands, apparently trying to bend it. Holder's niece Mary comes at the sound of all the shouting and, seeing the damaged coronet, faints dead away. Three beryls are missing from it. In a panic, he travels to see Holmes, who agrees to take the case.