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.
Deanna volunteers to help Maques and Hedril, but it's clear that Lwaxana is better for the task. She still uses her telepathy occasionally, and soon falls into a coma. Dr. Crusher cannot figure out what is causing it, as the telepathy-chemical depletion would not cause a coma. Deanna, with Maques' help, decides to go into her mother's brain telepathically in an attempt to help her.
Once inside her mother's thoughts, she has to fight Lwaxana's desire to keep a secret. Deanna makes her way through several obstacles, including a delay tactic in the form of her own deceased father. Finally, after she jumps off into illusionary space, she finds her mother and learns that she once had a sister named Kestra, her mother's "precious one". Kestra had died in a drowning accident and Lwaxana blamed herself. Hedril's appearance reminded Lwaxana of Kestra and brought back the painful memories of her death.
After confessing to Deanna, Lwaxana awakens from her coma. Mother and daughter have a heartfelt talk about Kestra.