Holmes Epiphany 

Noun, e.g., In the episode “The Last Bus to Woodstock,” Chief Inspector Morse has a Holmes epiphany:

Sgt. Lewis: “See you in the morning, sir.”
Chief Inspector Morse: “What did you just say?”
Sgt. Lewis: “Just see you in the morning.”
Chief Inspector Morse: “Sylvia Cane was seeing someone in the morning! . . .”

Definition: Dialogue in which one character, usually the sidekick of the movie’s sleuth, makes an innocuous remark, prompting the sleuth to ask him to repeat it, the words triggering for the sleuth an association that reveals the solution to the mystery..

History: If not invented by the screenwriters of Universal’s Sherlock Holmes movies, this plot device was certainly popularized by them. First, it gave the clueless Dr. Watson (Nigel Bruce) marginal credit for helping Sherlock Holmes (Basil Rathbone) solve cases. More important, it cued the audience that the scene to come would disclose the villain, known in Hollywood vernacular as the "payoff."

Until the Holmes series, the most popular mysteries were “The Thin Man” (1934), “After the Thin Man” (1936), and "Another Thin Man," (1939); seven Perry Mason movies, such as "The Case of the Howling Dog" (1934), "The Case of the Velvet Claws" (1936), and "The Case of the Black Cat" (1936); and several Charlie Chan films, such as “Charlie Chan in London” (1934), "Charlie Chan at the Opera" (1936), and "Charlie Chan in Reno" (1938). All used the gathering of suspects to introduce the payoff. This formula didn’t suit Holmes’ plots, whose mysteries often involved the why and/or how of crimes rather than the who. The Holmes epiphany has since been employed in every kind mystery, and particularly by writers of British and American television mysteries.