Stardate 2713.5: The Enterprise answers an old-style distress signal from an unknown planet and discovers a world that is virtually identical to Earth of the 1960's. Beaming down, the landing party finds a civilization populated only by children. After McCoy is attacked by a diseased humanoid, the landing party manages to corner and question a young female, Miri, and quickly learns that 300 years ago scientists attempted a life-prolongation project which instead created a virus that wiped out the adult population. The children, however, were altered in that their aging is slowed considerably until they reach puberty where the virus rapidly destroys their changing bodies. Soon thereafter, all members of the landing party contract the virus except for Spock who becomes a carrier. McCoy begins work on an antidote while Kirk tries to contact the other children, all of whom are afraid of adults after witnessing their violent behaviors when affected by the virus. Miri, however, becomes interested in Kirk and spends more time with him until she learns that Yeoman Rand has had an interest in the captain as well. She then helps Jahn steal the landing party's communicators and abduct Rand. When Kirk shows Miri that she too is contracting the disease she agrees to help him recover Rand and the communicators. Meanwhile, McCoy and Spock have developed what might be the antidote, but without the communicators to interact with the ship's computers they are unable to know if the antidote will cure or kill them. Frustrated, McCoy tests the antidote on himself and fortunately becomes the first of the landing party to be free of the disease. With the virus eliminated, the Enterprise leaves the planet in the hands of Federation truant officers who will help the inhabitants rebuild their civilization.