 The Enterprise visits a Earth-duplicate planet that s been in ruins for 300 years. Amazingly, they find it populated with children who are as old as the ruins. Then, the landing party discovers why they re so old: a disease that vastly slows the aging process, but then quickly kills once the person reaches puberty. It s also infected the landing party, who cannot return to the Enterprise and only have a week to live. The children are very distrustful of the "grups" (grown-ups). They use one of their own, a girl named Miri, to gain the landing party s trust, then betray them and ruin whatever hopes they have to find a cure.