"Elementary, Dear Data" is a pretty good Star Trek: TNG episode - but it is a clear rip-off of "The Big Goodbye."
Season one episode "The Big Goodbye" (that sees Picard, Data, & Dr. Crusher stuck in the Holodeck with the safety protocols deactivated) is a Star Trek: TNG episode that is not only a very good episode but also a Peabody & Emmy Award winner. Season two's "Elementary, Dear Data" attempts to mimic the success of "The Big Goodbye" by creating a hostile Holodeck and similar themes espoused in "The Big Goodbye."
The similarities are astounding: in "The Big Goodbye," Picard is thrilled to play the part of Dixon Hill - in "Elementary, Dear Data," Data throws himself into the role of Sherlock Holmes (although, one would think that if Data is a Holmes expert he would have played him more like Robert Downey Jr. did). In "The Big Goodbye," holographic antagonist Cyrus Redblock realizes that he is on a ship and questions his own existence - in "Elementary, Dear Data," the holographic antagonist Professor Moriarty realizes he is on a ship, he is alive, and threatens to destroy the crew if he is not kept alive.
Even though Wesley Crusher does not come in and save the day AGAIN (thankfully) and despite the many obvious similarities between both episodes, "Elementary, Dear Data" is not as good as "The Big Goodbye" - if not just because of the docked points for originality. "Elementary, Dear Data" does not have the style, energy, or character that "The Big Goodbye" had. It tries to be stylish but looks washed out and simply cannot compete with the film noir style of "The Big Goodbye." It attempts to be character-oriented as Geordi and Polaski try to see if Data can actually solve a mystery (which he has done time and time again on the freakin' bridge of the Enterprise) but then pitters out when it is discovered that Moriarty is alive. Finally, the scene in "The Big Goodbye" featuring Cyrus Redblock's realization of his own existence and subsequent "death" is far more poignant than the desperation in Moriarty's eyes in "Elementary, Dear Data."
I am not a huge fan of the Holodeck episodes and while "Elementary, Dear Data" is a generally fun time, it's not a particularly strong episode.