The Scalosians are a sapient humanoid species from the planet Scalos; whose metabolism works at an incredibly accelerated rate, rendering them invisible and undetectable by 23rd century tricorders.
From a Scalosian's point of view other species appear like statues, and even phaser shots move in slow motion. Their accelerated biochemistry which causes them to experience days in minutes was caused by some sort of industrial disaster involving radiation which modified their biology and also rendered the vast majority of them infertile.
As their population decreased, they were forced to lure passing starships with fake distress signals and kidnap some of the crew (the abductee's metabolism being likewise modified by chemical means to match the Scalosians', causing the impression that they simply vanished) in order to procreate with them. As of 2268, their population had declined to a mere five individuals. In that year, after a failed attempt to abduct James T. Kirk and Compton in such fashion (and which resulted in Compton's death), the Federation placed Scalos in quarantine to prevent other starships to fall victim to them, and the Scalosians presumably went extinct at some point due to sterility.
- In this episode, Spock and McCoy are able to create an agent which reverses the acceleration and returns Kirk to normal. Why they didn't offer it as a cure to the Scalosians remains unclear and arguably out of character for them. Also, the idea of the Scalosians being able to alter other species' metabolism to match their own without making them sterile as well would most surely imply that they should have enough knowledge about their condition to be able to cure their own accelerated living and/or sterility. This could be explained in that it is stated that accelerated Humans don't live long (even by Scalosian standards) and Compton indeed dies as a result of his transformation, implying that while the Scalosians are able to change another being's metabolism without compromising fertility, they are unwillingly to try on themselves for fear of the collateral effects.