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After purchasing the diary of an alleged suicide victim at an auction, Della (Barbara Hale) is surprised when someone offers her $1000 for the book. Turning down the offer, the intrigued Della begins paging through the diary, ultimately arriving at the conclusion that its author did not kill herself, as originally reported. Della brings the diary to the attention of her boss Perry Mason (Raymond Burr)--thereby setting off a chain of strange and sordid events, culminating in Perry's defense of Josephine Kempton (Lurene Tuttle) on a murder charge. The "grinning gorilla" mentioned in the title is played by celebrated stunt man and simian impersonator Janos Prohaska. This episode is based on a 1952 novel by Perry Mason creator Erle Stanley Gardner.