The crew finds a duplicate Earth where only children survive, and a disease is killing the landing party.
The Enterprise finds a planet that is an exact copy of Earth in the twentieth century. The only survivors are children, who age very slowly, and adults who fell to a deadly disease long ago. When the landing party is infected, a cure must be found against the clock.
Miri is memorable for its eerie setting, a world of empty streets and playground rhymes, and for its guest turn by Kim Darby. The child actors' performances give the story an unsettling air, and it is often discussed as an early example of the series' fondness for strange parallel worlds.