"Elementary, Dear Data" is the third episode of the second season of the American science fiction television series Star Trek: The Next Generation, the 29th episode overall. It was written by Brian Alan Lane and directed by Rob Bowman. It was originally released on December 5, 1988 in broadcast syndication.

Set in the 24th century, the series follows the adventures of the crew of the Federation starship Enterprise. In this episode, a holographic adversary manages to take control of the Enterprise. The plot line from this episode was continued in the sixth season episode "Ship in a Bottle".

As the Federation starship Enterprise, under the command of Captain Jean-Luc Picard, waits to rendezvous with the USS Victory, Chief Engineer La Forge and Commander Data go to the Holodeck to recreate a Sherlock Holmes mystery. Data, who has read all of the Holmes stories, recognizes and solves the mystery within minutes. Frustrated, Geordi storms out of the holodeck, leaving Data confused. In Ten Forward, Geordi explains that the fun is in solving the unknown; Data does not understand. Overhearing their conversation, Chief Medical Officer Dr. Pulaski asserts that Data is incapable of solving a mystery to which he does not already know the outcome. Data accepts Dr. Pulaski's challenge and invites her to join them on the Holodeck. There, Geordi instructs the computer to create a unique Sherlock Holmes mystery with an adversary who is capable of defeating Data.

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