Troi's freewheeling mother Lwaxana upsets Deanna and Worf when she introduces Worf's son to her rather hedonistic lifestyle.
After destroying a deadly asteroid in the Pelloris Field, the U.S.S. Enterprise heads toward the Moselina system unaware that a cloud of strange particles have attached themselves to the ship's hull. As they travel, Deanna Troi's mother Lwaxana transports aboard with an unusual announcement - she is planning to get married on the Enterprise to a man that she has never met. Troi finds the news disturbing, but Lwaxana laughs at her "motherly" concern.
Troi has been counseling Worf and his son Alexander, who have been clashing over the boy's responsibilities. Soon, Lwaxana meets Alexander and takes a liking to him. She persuades him to skip his appointment with Troi and accompany her to the holodeck instead. There, she takes him for a visit to a colony of artists, poets and free thinkers, and to a mudbath. Troi and Worf, meanwhile, begin a search for the missing boy that leads them to the holodeck.
An angry Troi asks her mother to stop interfering with Alexander's upbringing. The subject changes to Lwaxana's upcoming wedding, and Troi is shocked to learn that her independent-thinking mother plans to forgo the Betazed custom of getting married in the nude and wear a wedding dress provided by her bridegroom instead. Meanwhile, minor malfunctions begin to occur aboard the ship, and Geordi and Data discover that mechanical parts of the ship are being transformed into a gelatinous substance. As they report their findings to Picard, Riker and Worf, red alert sounds as more crucial systems begin to fail.
Later, Lwaxana's intended, Minister Campio, transports aboard along with his pompous Protocol Master. Lwaxana is a bit taken aback by just how stuffy her husband-to-be is, since the compatibility profile that matched them did not alert her to how major their differences are. She becomes bored with the complicated wedding plans and heads back to the holodeck with Alexander, much to everyone's dismay. Back in engineering, Geordi discovers the strange particles that have attached themselves to the ship and realizes they are at the root of the mechanical problem.
Malfunctions interrupt Lwaxana and Alexander's holodeck visit, and she calmly leads the boy out of danger. Meanwhile, Geordi and Data tell Picard that the particles that are destroying the ship are parasites that turned to the Enterprise after the asteroid they fed on was destroyed. They must return to the Pelloris Field in order to get rid of the particles, but time is running out. The parasites quickly overtake the life support systems, and the entire crew, with the exception of Data, loses consciousness as the oxygen runs out. Racing against the clock, Data reaches the Pelloris Field and beams the particles toward a new asteroid. With everything back to normal, Lwaxana's nuptials begin. They come to an abrupt halt, however, when she walks down the aisle naked according to her traditions, sending her stuffy bridegroom and his Protocol Master scurrying for home. Troi is pleased that her mother stuck to her guns, and later brings Worf to join Lwaxana and Alexander for a last trip to the mudbath.
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