While this one does not usually list among "Trek" favorites, it is one of mine. 40's star Ellen Geer guests as "Dr. Kila Meer," a much respected scientist on board the Enterprise to study the mysterious "crystal entity" that had appeared in an earlier episode that introduced Data's brother, Lor. Though Dr. Meer appears to be genuinely interested in the crystal life form, she hides an ulterior motive: seeking retribution for the death of her son at the hands of the entity.
With the assistance of the crew, specifically Data, Dr. Meer is able to carry out her plan, much to the dismay of the android and the captain.
With the assistance of the crew, specifically Data, Dr. Meer is able to carry out her plan, much to the dismay of the android and the captain.