When the Enterprise answers a distress call from the U.S.S. Lantree, their investigation leads them to the Darwin Genetic Research Station on Gagarin IV, where Dr. Pulaski contracts an aging-disease from genetically-engineered human children.

Diana Muldaur (Dr. Katherine Pulaski)
Patricia Smith (Dr. Sara Kingsley)
Colm Meaney (Transporter Chief O'Brien)

J. Patrick McNamara (Captain Taggert)
Scott Trost (Ensign)
George Baxter (The "Child")

• Darwin Station has created genetically engineered humans, even though it has been established that this is illegal in the Federation.
  
• This is the first episode in which the Transporter Chief played by Colm Meaney is given a name: O'Brien.
  
• The Enterprise destroys the U.S.S. Lantree NCC-1837 to prevent spread of the virus.
  
• The Miranda-class U.S.S. Lantree is a re-use of the U.S.S. Reliant model from Star Trek II: The Wrath Of Khan.