Mymble has found a ring on the ground with a big, sparkling stone and gives it in to the Inspector. When no one comes forth to claim it he gives it to her and she is very, very happy, deciding it must be a diamond. She parades it all around Moominvalley, until she is informed by Fillyjonk that if one has a huge diamond like that one must dress accordingly. Not only that, but one must behave accordingly and live accordingly and even though Mymble is helped by Little My and Stinky she soon finds that although diamonds may be a girl's best friend, they should be so in moderation..