Geordi asks the holodeck for a Sherlock Holmes foe who can outwit Data.
Data and Geordi enjoy a holodeck recreation of a Sherlock Holmes adventure, but Data solves it too easily, since he knows the stories. To challenge him, Geordi asks the computer to create an opponent who can defeat Data. The result is Professor Moriarty, a holographic character who becomes aware of his own existence.
The episode uses its comic premise to ask an interesting question about artificial life. Moriarty threatens the ship by taking control of it, and the crew has to find a way to resolve the situation without destroying him. It was popular enough that Moriarty returned in a later season. It is also known for Data's fondness for Holmes, and for Dr. Pulaski's friction with him.