A 1971 film starring Creator/GeorgeCScott and Joanne Woodward. Adapted from a stageplay. The main character believes he is Franchise/SherlockHolmes after the death of his wife leaves him with severe paranoia. Dr. Mildred Watson is an analyst caught up in the man's brother's scheme to control his money through having him committed, while at the same time the brother is being pursued by a shadowy blackmailer in sunglasses. The film spoofs classic SherlockHolmes tropes and ponders the popular themes of insanity vs society's consensus reality.

Yes, [[Music/TheyMightBeGiants the band]] named themselves after this story.
!!The film contains examples of:
* AwLookTheyReallyDoLoveEachOther: Initially they are very antagonistic towards each other, he annoyed and fed up with her attempts to 'cure' him and her exasperated by his maniacal antics.
* ClapYourHandsIfYouBelieve: The story plays with the idea of how groups of individuals who have chosen to embrace a delusional reality can create their own consensus reality which just might, depending on how you interpret the ending, become real.
* TheCuckoolanderWasRight: For all his apparent insanity, the presence of the mysterious blackmailer suggests that Justin may have stumbled onto a genuine conspiracy of some sort after all.
* DetectiveDrama: Played with. The main character thinks he's Holmes, but all the deductions he makes are patently ridiculous (even if the story allows them to be true), the clues he constantly searches for are more like the manias of a schizophrenic finding meaning and messages in completely unconnected and unintentional words, and he commits the cardinal sin of theorizing without data (he posits the existence of Moriarty without any evidence). However, the presence of the plotting brother and the shadowy blackmailer turns this into an interesting, if off-kilter, example.
* EveryoneCallsHimBarkeep: Most of the people close to him call him Justin Playfair and he himself admits that "scarcely anyone" calls him Holmes, but most of the city crazies seem to know him as Sherlock Holmes.
* FunnySchizophrenia: If you don't believe that Justin is right, his crazy antics come off this way, such as suspiciously observing pigeons with a magnifying glass.
* IntimatePsychotherapy: The Doctor assigned to "cure" him of his alleged monomania later falls in love with him and embraces his delusions by taking on the persona of Watson.
* MeaningfulName: Dr. Watson. Might also be a bit of 'nominative determinism' in that he seems to treat her as TheWatson because she ''is'' "Dr. Watson".
* MindScrew: The ending leaves one wondering, 'what just happened?' Did the pair see Moriarty? Did they get run over by a car, smiling wide-eyed all the while? Or was that the spotlight of the police?
* MilkManConspiracy: Justin/Holmes points to such disparate incidents in the newspapers as people dying from crossing the street, grandmas being raped and beaten, and excursion boats exploding as evidence of Moriarty's near omnipresent nefarious influence.
%%* [[spoiler:NoEnding]]
* NoMeansYes:
-->'''Justin/Holmes:''' "Did you enjoy your first detective work?"\\
'''Watson:''' "No." \\
'''Justin/Holmes:''' "Come on, I know girls--no means yes."
* RagtagBunchofMisfits: The epic march near the end gathers together all of the city crazies as well as most of the people they've helped throughout the film, all marching in tandem alongside Justin/Holmes and Watson. PlayedForLaughs when he asks them, "Anyone who knows why we are here raise your hand." No one does.
* TitleDrop: Justin's speech in the taxi about why the Literature/DonQuixote spirit helped carry forward history and progress, by looking at things and thinking of what they might be--for example windmills because...''They might be giants''.
* TheMoriartyEffect: As a Sherlock film, this is predictable enough
* WindmillCrusader: Justin, hence the title.