Justin Playfair (George C. Scott) is a millionaire who retreats into fantasy after the death of his wife, imagining himself to be Sherlock Holmes, the legendary fictional detective. Complete with deerstalker hat, pipe and violin, he spends his days in a home-made criminal laboratory, constantly paranoid about plots hatched by his arch-enemy, Professor Moriarty. When his brother (Lester Rawlins) places Justin under observation in a mental institution, and conspires with his former business associate to get power of attorney, Justin attracts the attention of Dr Mildred Watson (Joanne Woodward), a psychiatrist who becomes fascinated by his case. After Justin demonstrates a knack for Holmesian deduction, the institution releases him, and Watson meets him at his home. Playfair is initially dismissive of Watson's attempts atpsychoanalyzing him, but when he hears her name, he enthusiastically incorporates her into his life as Doctor Watson to his Holmes. The duo then begin an enigmatic quest for Moriarty, with Playfair/Holmes following all manner of bizarre and (to Watson) unintelligible clues, and the two growing closer to each other in the process.