The U.S.S. Enterprise welcomes a delegation of the Cairn, a telepathic species that has no concept of spoken language. Since an interpreter with strong telepathic ability is needed, Troi's Betazoid mother, Lwaxana, is recruited for the job, and immediately sets about trying to start a romance between her daughter and Maques, the visiting group's senior diplomat. Troi's initial anger turns to concern when, after confronting Lwaxana, her mother bursts into tears. Later, Maques apologizes to Troi for their awkward first meeting, and also tells her that he has sensed that a part of Lwaxana's mind is hidden and dark. Troi is shaken up the next day when Lwaxana angrily admonishes Riker for touching Troi. The counselor takes her mother to see Beverly, who finds that Lwaxana is low on psilosynine, a neurotransmitter involved in telepathy. Lwaxana is ordered to refrain from using telepathy, and Troi offers to help in her work with the Cairn. However, no sooner are they back on the job than Lwaxana defies her doctor's orders and communicates with Maques, collapsing soon afterward.
In Sickbay, Beverly sadly tells Troi that her mother's brain has shut down. Picard and Troi approach Maques for help, and he relays his complicated thoughts to Troi telepathically, telling her that Lwaxana may have suffered some emotional event in the past that her meta-conscious mind, the system Betazoids use to protect themselves from psychic trauma, can't tolerate. Troi tries to reach Lwaxana telepathically, but hears only a faint cry for help. She falls asleep at her mother's bedside, and awakens to find Maques standing above Lwaxana's bed, engaged in an attempt to reach her.
Maques explains that he was trying to help Lwaxana, who he feels has retreated into her meta-conscious mind. He offers to help Troi search for the trauma that has damaged Lwaxana by providing a telepathic "bridges that will allow Troi to access and interpret the images in her mother's psyche and hopefully learn what caused its collapse. She reaches Lwaxana, who is standing with Hedril, a young Cairn girl who is part of the visiting delegation. The procedure then ends abruptly when Lwaxana angrily orders Troi to go away.
Realizing that the girl may hold the key to Lwaxana's problem, Troi meets with Hedril, who says she knows she makes Lwaxana sad. Troi, Beverly, Picard, and Data discuss the images Troi saw in her mother's psyche, but are unable to determine what they mean or what she may be hiding. Troi then turns to her mother's recent journal entries for answers, but is interrupted by Picard, who warns her that Lwaxana is deteriorating. Together, they look back at entries beginning the year of Lwaxana's wedding, and are shocked to discover that she deleted seven years of entries that began shortly after her marriage and ended soon after Troi's birth. Sure that this is a clue, Troi decides to enter her mother's mind again.
This time, Troi encounters Hedril a second time and chases her, but the girl suddenly vanishes. Troi ends up in the arboretum, watching her mother, her father, Hedril and herself as an infant during an outing. Lwaxana refers to Hedril as Kestra, and through their interaction, Troi realizes that this girl who she has never heard of must be her sister. Lwaxana's pain increases as Troi presses her to continue, but with her daughter's encouragement, she is finally able to reveal that Kestra died that day in an accident — an accident for which Lwaxana blames herself. Troi is able to help her mother say goodbye to Kestra and let go of her guilt. Later, after the "dream" is over, she lovingly gives her mother a picture of Kestra, baby Deanna, and her father so that Lwaxana can always remember them.