Elementary, Dear Data

La Forge is frustrated with Data's ability to solve holodeck mysteries too quickly when playing Sherlock Holmes, ruining the element of suspense in the story. On a suggestion, he accidentally creates a villain who is capable of really challenging Data (in the form of Moriarty, Holmes' greatest adversary), which puts the Enterprise into a lot of trouble. Detective stories tend to make for great episodes, so why not throw in the greatest fictional detective of all time into a story? The plot is simply fantastic and the twist feels so obvious that you feel like you should have picked it up much before then. This episode again demonstrates how using sci-fi to alter the parameters of our universe can be used to tell a great story.