They Might Be Giants (1971) on Netflix

Plot summary

Justin Playfair lives as Sherlock Holmes, deerstalker hat and pipe included, and works in a homemade criminal laboratory on plots he credits to Professor Moriarty. The retired judge lost his wife and now follows clues across the city in that role. His brother Blevins wants him committed so that he can gain power of attorney over Justin's finances, and a criminal Blevins owes money wants Justin dead. The commitment papers need the signature of psychiatrist Dr. Mildred Watson, who insists on a proper interview and is fascinated by Justin's gift for deduction. He walks out of the institution amid the confusion. Watson visits his home to attempt treatment, and when he hears her name he adopts her as Dr. John Watson, the chronicler of his cases. The two begin a search through the city for Moriarty.