A distress signal draws the crew of the Enterprise into a perilous situation (for a change). This time, the message emanates from a planet that is, for some unknown reason, an exact duplicate of Earth. Beaming down to the planet's surface, Kirk, Spock, Bones, Yeoman Rand, and two red shirts find a world in ruins, the only inhabitants a gang of almost feral children. They eventually discover the reason for the state of the planet: three hundred years earlier, scientists attempted to develop a way to prolong life, an experiment that went horribly wrong, achieving the desired results in the pre-pubescent members of the population, but causing a disease in the adults that resulted in madness and death. Kirk and company soon start to show signs of the sickness, and attempt to rustle up a cure, but their progress is hampered by the children (who are, in reality, several centuries old), who view adults (or 'grups' as they call them) as a threat. Miri (Kim Darby) is the only one to help the crew, the preadolescent girl having fallen for the dashing captain.
Miri is an entertaining episode, but it's a problematic one. The whole doppelganger Earth aspect is totally pointless - no explanation is given for this one-in-a-gazillion occurrence. The crew beaming down to the the exact location where the virus was developed is far-fetched in the extreme. The entire crew leaving their vital communicators unattended is an obvious and irritating plot contrivance. Kirk and friends arriving on the planet just as the kids' food supply is about to run out-what are the chances? Michael J. Pollard in his late twenties as one of the 'children' - give me a break! And as for Kirk's inappropriate flirtatious manner with Miri... it's such uncomfortable viewing that it's little wonder that the ever-cautious BBC played it safe by not airing this episode for a couple of decades.