star trek question

what was the episode where they were under mind control

"Platos Stepchildern?" where "the Greeks" made people do things?
TOS or TNG? Do you mean telepathic control, control by a parasitic life form, or perhaps Borg Implants? "Landru" did this, as did the creatures in "Catspaw", operation Anihillate had the "pancake creatures" which affected the CNS. The episode "Return to Tomorrow" where Sargon and the others transferred their consciousness into Kirk, Spock., and ??? can't remember her name would also be considered mind control. As would Menagerie, "The Cage", "Where no man has gone before". Too many to count. Mind Control was as NobodyInParticular said, a frequently used plotline device in many of the series.
Mind control appears in many Star Trek episodes.

Perhaps you mean Return of the Archons? (also on IMDB)