Growing old is bad for you! Broadcast September 23rd 1967
On mission date 1647.2, the Endeavour, enters the Ramsey system in order to chart its jump points. As they go about doing this Colt picks up a distress signal from the third planet. Decker decides to investigate. On arriving at the planet they find that it is very earth like but seems to be in the middle of a glacial period and so it is a few degrees cooler than earth. Reynolds jokes that the Andorians would love this planet as a tropical paradise.
Further investigation finds the ruins of an industrial civilisation whose buildings seem remarkably similar to those of mid to late twentieth century America. A landing party consisting of Reynolds, Spock, Fynely, Stein, Rand and two security personnel, beams down to investigate. Upon their arrival, they find that the planet seems abandoned. As the team curiously inspects a tricycle, they are attacked by a ragged man, who seems deranged and also shows incredible strength. Reynolds strikes the man three times, the man has a seizure and dies. The security guards then spot another figure running into a building.
The team chases the figure down, discovering that it's a terrified girl, who appears physically normal. She identifies herself as Selli (guest star Jenny Agutter), and when asked why she ran away from them, she tells them because they are "grunpies" (a contraction of "grown ups"), and the grunpies killed and hurt the children before they died. When asked where her family are, Selli tells them that she is an "Vivor," and she and her friends are the "only survivors” left as all the adults are dead.
Soon the landing party starts to notice that their tempers are shortening and that they are becoming increasingly irrational; Spock however remains immune. Selli informs them that these are the first signs of the disease, and they will soon become like the other adults. The team searches an abandoned hospital for clues to the mysterious condition. They discover the disease infects those who have reached even the earliest stages of puberty. It is an accidental side-effect of an experiment to prolong life; the technique works on children, but when they reach puberty, they enter a short period of violent rage and then die. Shockingly, they learn that the children are over 100 years old, having aged only one week for every decade.
Spock also discovers that once the disease starts, they only have 7 days to live. Even though he seems immune to the disease, he believes he is still a carrier and could infect the Endeavour if he returns.
Meanwhile, the rest of the hiding children, who do not trust these new grunpies, decide to meddle with their plans. Their leader, a boy named Domnik (Peter Firth), steals the landing party's communicators, which renders Fynely’s and Stein’s search for a cure nearly impossible without assistance from the Endeavours computers. Selli however, doesn't agree with the other children's mischief and stays near Reynolds, on whom she appears to have a crush. However Sellii becomes jealous of Rand and runs away to scheme with her friends. The children devise a prank and kidnap Rand.
Reynolds confronts Selli, who tells her that she and the others will eventually contract the disease just like the grunpies if they don't help him find a cure. Sellii takes Reynolds to the schoolhouse where Rand is being held captive. He confronts the children and tries to get through to them that none of this is a game. At first the children don't listen and continue to harass him, encouraged by Domnik. They become increasingly menacing until Domnik finally beats Reynolds nearly senseless with a wrench. This action shocks the Vivors as Domnik is now acting like a grunpie. Domnik screams and runs away.
Reynolds and Rand round up the children and return to the hospital with the communicators, but finds that Spock has previously found that Fynely and Stein, unable to accurately test their experimental serum, has injected Stein with a dose and he had collapsed to the floor. Soon however, he revived showing no ill effects and when scanned by Fynely’s medical scanner was cured of the disease. The serum is found to be both safe and effective. After curing the landing party and the children, Decker informs the Fleet to send teachers and advisers, to help the children start their lives over again. He also comments that Humans seem to be far more widespread than they could ever have imagined and that there may be yet more of the “Lost” to be found.