Created by Gene Roddenberry, the science fiction television series Star Trek (which eventually acquired the retronym Star Trek: The Original Series) starred William Shatner as Captain Kirk, Leonard Nimoy as Mr. Spock, and DeForest Kelley as Dr. Leonard "Bones" McCoy aboard the fictional Federation starship USS Enterprise. The series originally aired from September 1966 through June 1969 on NBC.
This is the first television series in the Star Trek franchise, and comprises 79 regular episodes over the show's three seasons, along with the show's original pilot episode, "The Cage".
After the show's cancellation, Paramount Television released Star Trek to television stations as a syndication package, where the series' popularity grew to become a "major phenomenon within popular culture." This popularity would eventually lead the Star Trek franchise to expand its catalog to include five more television series and twelve Trek motion pictures.
On stardate 2713.5, the Federation starship USS Enterprise, under the command of Captain James T. Kirk, follows an old planetary distress call leading to a planet that resembles Earth in every detail.
Kirk assembles a landing party consisting of himself, First Officer Spock, Chief Medical Officer Dr. McCoy, Yeoman Janice Rand, and two security personnel, to investigate this remarkable find. Upon their arrival, they find the planet resembles Earth of 1960, but seems abandoned. As the team curiously inspects a tricycle, they are attacked by a ragged man who seems infected by a horrible disfiguring mutation, but shows incredible strength. Kirk 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 Miri (Kim Darby), and when asked why she ran away from them, she tells them because they are "grups" (a contraction of "grown ups"), and the grups killed and hurt the children before they died. When asked where her family are, Miri tells them that she is an "Onlie," and she and her friends are the "only ones" left as all the adults are dead.
Soon the landing party starts to notice painful blue sores are forming on their bodies; Spock however remains immune. Miri 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 only those who have reached 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 300 years old, having aged only one month's time every century.
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 Enterprise if he returns.
Meanwhile, the rest of the hiding children, who do not trust these new grups, decide to meddle with their plans. Their leader, a boy named Jahn, steals the landing party's communicators, which renders McCoy's search for a cure nearly impossible without assistance from the Enterprise computers. Miri however, doesn't agree with the other children's mischief and stays near Captain Kirk, on whom she appears to have a crush. However, when Yeoman Rand becomes hysterical over their impending fate and Kirk tries to console her, Miri becomes jealous and runs away to scheme with her friends. The children devise a "foolie" prank and kidnap Rand.
Returning later, Miri is confronted by Kirk, who tells her she and the others will eventually contract the disease just like the grups if they don't help him find a cure. In fact, he grabs Miri's arms and shows her the blue sores that are already forming on her skin.
Miri takes Kirk to the schoolhouse where Rand is held captive. He confronts the children and tries to get it through their heads that none of this is a game. At first the children don't listen and continue to harass him, encouraged by Jahn. They become increasingly menacing until one of them finally beats Kirk nearly senseless with a wrench. Bruised and bloodied, Kirk implores them to think of the youngest onlies, who will be left without resources when the older ones are dead. He warns them that the stores of food and supplies are nearly depleted. He also points out that the children have hurt him and now have blood literally on their hands, exactly like the grups they are afraid of. "I'm a grup," he says, "and I want to help."
Kirk rounds up the children and returns to the hospital, but finds that Spock has previously found that McCoy, unable to accurately test his experimental serum, has injected himself with a full dose and collapsed to the floor. Soon however, his sores fade away. The serum is found to be both safe and effective. After curing the landing party and the children, Kirk informs Starfleet to send teachers and advisers to the duplicate Earth, to help the children start their lives over again.