A virus-like T-cell disease causes the crew to exhibit strange behavioral changes, eventually regressing into primitive creatures.

Dwight Schultz (Lt. Reg Barclay)
Patti Yasutake (Lt. Alyssa Ogawa)
Carlos Ferro (Ensign Dern)

• Data's cat Spot is pregnant. (The cat was previously male.)
  
• Alyssa tells Dr. Crusher and Data that her and Andrew Powell are also having a baby, after announcing that they were getting married in episode 167 "Lower Decks".
  
• This is the first episode where we see Deanna on bridge duty since her promotion to Commander in the episode "Thine Own Self".
  
• Dr. Crusher is incapacitated by Worf fairly early on to give Gates McFadden less on-screen time for directing.
  
• Dr. Crusher gives Barclay a synthetic T-Cell to activate an intron (dormant gene) to fight his flu, but it mutates into an intron virus and infects the entire crew. Dr. Crusher names the disease "Barclay's Protomorphosys Syndrome."
  
• Data uses Alyssa's amniotic fluid to create an retro-virus antidote.
  
• Continuing their romance, Worf and Deanna have a lunch date in Ten Forward.
  
• Worf regresses into a Klingon animal, who sees Deanna as his mate. Deanna regresses into an amphibian. Barclay regresses into an arachnid. Riker regresses into a proto-human australopithecine. Picard begins de-evolving into a monkey (lemur or pigmy marmoset).
  
• Barclay was introduced in Season 3's "Hollow Pursuits".
  
• Dr. Crusher mentions Vulcan Dr. Selar, who has only made one on-screen appearance, in the second season's "The Schizoid Man".
  
• This episode won an Emmy Award for Outstanding Individual Achievement in Sound Mixing. It was also nominated for Outstanding Individual Achievement in Sound Editing and Outstanding Individual Achievement in Makeup. (It did not win any fan awards.)