Miri
Kim Darby

Jahn
Michael J. Pollard

Lt. John Farrell
Jim Goodwin

The Enterprise, hundreds of light years from home, goes to investigate a distress call transmitted in Earth-type radio signals. On arriving at the apparent source they find a planet which appears to be an exact duplicate of Earth. A landing party is beamed down to investigate further but is immediately attacked by a madman. As the man dies, McCoy is horrified by the fact that he appears to age incredibly in just a few seconds.

The buildings they explore seem to be deserted until they come across a young girl called Miri. They discover that a plague of some sort has killed off all the adults and that all of them in the landing party, except for Spock, have also been infected. They persuade Miri to take them to an abandoned hospital where, in a laboratory, McCoy discovers files relating to a project on life prolongation.

Spock surmises that the unseen effect of this is to prolong childhood for hundreds of years, finally causing madness, rapid ageing and death when affected by the chemical and hormonal changes during the onset of puberty. He and McCoy have a limited amount of time to find a cure before they all succumb. Other children on the planet, afraid of grown-ups because of their deranged behaviour before death, meet secretly and plot to steal the landing party's communicators.

McCoy and Spock are unable to find an antidote without the help of the Enterprise's computer and arguments start to break out as the disease takes hold and nerves get frayed. The crewmembers start to break out in red-blue blotches and Kirk tries to console Janice Rand when she discovers them on her legs. Miri has developed a crush on Jim and, in her jealousy, arranges for the children to kidnap her.

Kirk manages to rescue Janice and also manages, eventually, to persuade the children that he and the Enterprise can help them. He gets the communicators back from the children and races back to the lab only to find that McCoy has already injected himself with a possible antidote, which has rendered him unconscious. The injection, however, works and the members of the landing party are cured. The Enterprise leaves the planet after arranging Federation assistance for the 300-year old children.