自从61岁的普莱斯伯利教授狂热地爱上一个少女后,每隔九天,他就会被家里的狼狗狠狠地咬一次。而更令教授的女儿和助手惊骇的是:被狗咬的当天深夜,教授总是偷偷地溜出房间,在楼道和窗户上四处爬行……Sherlock Holmes is asked by Jack Bennett to investigate after his fiancée Edith Presbury sees the shadow of a man in her window in the middle of the night. The problem is that her bedroom is on the second floor and there is no way for any man to have been able to get to that window. Edith is the daughter of a notable natural scientist, Professor Presbury, a widower who is engaged to be married to a much younger woman, Alice Morphy. The Professor is quite displeased at hearing that Bennett, who also happens to be his assistant, has hired the Great Detective and Holmes and Watson are unceremoniously escorted off the property.When Holmes reads of a rash of thefts from zoos - all monkeys and apes - he slowly begins to unravel the mystery of the shadow Miss Presbury saw in her window.