Red-Headed League

From Memory Alpha, the free Star Trek reference.

The Red-Headed League was a fictional organization from the Sherlock Holmes story The Red-Headed League. Ostensibly a society founded by American millionaire Ezekiah Hopkins for the betterment of red-headed men, the league was in fact an elaborate ruse masterminded by the criminals John Clay and William Morris, intended to keep red-headed pawnbroker Jabez Wilson occupied while Clay tunnelled from the basement of the pawn shop to the Coburg branch of the City and Suburban Bank, in order to steal French gold which was being kept there at the time. The plan was eventually foiled by Sherlock Holmes.

In 2365, Data, while masquerading on the holodeck as Sherlock Holmes, noticed a bell cord hanging in front of the door of the home of the Red-Headed League shortly after witnessing a thieving boy running away, and, from these three things, was able to deduce that Jabez Wilson was the intended target of a murder. Data however, had not come to this conclusion through deductive reasoning from the facts of the situation, but rather had simply compared the three different Holmes stories from which these elements came. (TNG: "Elementary, Dear Data")