Star Trek The Next Generation "Dark Page"
Lwaxana Troi returns to the Enterprise, this time as the teacher of an alien race learning to speak. Their native form of communication is telepathy, but they want to learn spoken language in order to interact with other races. Lwaxana comes aboard with Maques and his young daughter, Hedril, who is Lwaxana's star pupil.
The crew begins to notice that Lwaxana doesn't seem to be her normal flamboyant self. She's quiet and almost reserved. Deanna tries to figure out what's going on, and even has her mother examined in sick bay. Dr. Crusher determines that the neurotransmitter used for telepathy is sapped — Lwaxana must refrain from telepathy until it can regenerate. Then, a tragic event from the past closes Lwaxana down.