The Adventure of the Creeping Man (S02E06)

Inspector Lestrade discovers that Alice Murphy is set to marry their former teacher, Professor Jacob Presbury, who has become increasingly irritable and absent-minded. His daughter feels envious of their engagement, while family friend Trevor is overly possessive. Meanwhile, Holmes is looking into the theft of three monkeys from the Primate Shelter, which he soon learns were taken by Trevor. In a desperate bid to win Alice's affection, Trevor has transformed Presbury into an ape-like creature.