"Miri" is the eighth episode of the first season of the science fiction television series, Star Trek, that was first broadcast October 27, 1966, and repeated June 29, 1967. It was written by Adrian Spies and directed by Vincent McEveety.

In the plot, the Enterprise discovers an exact duplicate of Earth, where the only survivors of a deadly man-made plague are some of the planet's children.

The Enterprise answers an automated distress call from a planet resembling Earth in every detail. A landing party of Captain Kirk, First Officer Spock, Chief Medical Officer Dr. McCoy, Yeoman Janice Rand and two security personnel find an abandoned, 1960 Earth. When they examine a tricycle they are attacked by a strong, disfigured man. After Kirk hits the man three times, the man has a seizure and dies. A mysterious figure catches their attention, and they investigate.

They discover an adolescent Miri, who ran away from them because "grups" ("grownups") killed and maimed children before dying. She and her friends are "onlies", the only ones left.

The landing party, except for Spock, notice purple lesions on their bodies; Miri tells them that these are the first signs of the disease and they will soon become like the other adults. The party find a medical research laboratory and look through documents for clues to the disease, and discover that it is a side effect of a life-extension experiment, affecting those who have reached puberty; death follows a brief period of violent madness. The "children" are over 300 years old, aging one month every century.

Spock learns that when the disease begins, its victims have seven days to live. Although he is apparently immune, he considers himself a carrier who could infect the Enterprise if he returned.

The other children, mistrustful of the "grups", meddle with their plans. Jahn steals the landing party's communicators, rendering McCoy's search for a cure impossible without the Enterprise's computers. Miri opposes the mischief and remains near Kirk; when Yeoman Rand panics at their impending fate and Kirk comforts her, a jealous Miri runs away and schemes with her friends to kidnap Rand. Another girl goes insane and attacks Kirk before she collapses and dies. Jahn's line while banging the hammer on the table is "bonk, bonk on the head (repeated continuously)."

...
Wikipedia