Long Ago, two races rules over Earth: Humans and Monsters. One day, war broke out between the two races, after a long battle the humans were victorious, they sealed the monsters underground with a magic spell.
Many Years later a plague has striken humanity, the sickness left the humans almost extinct, to save their children some families sent them to the mountains, hoping that the disease would not reach them. Six families sent their children, but it was too late, they were already infected and none of them were seen again.
Each of these children fell into the underground where the monsters are sealed, the six children who happened to fall in the Underground, carried the virus with them and contaminated the monsters, but different from the humans, the monsters got their soul and mind infected, causing mental disturb everywhere.
When the monsters realized it, was already too late, king Asgore fearing the unknown disease, ordered his royal guard to eliminate any human who get in the undergound, But only when the sixth child was eliminated Alphys did realized, they needed the humans to develop a cure, so they had to wait for the next child to come, to continue his research.
Recently the seventh human has arrived in the underground, Frisk is your name and before his fall, Frisk was trying Frisk was trying to escape from the humans who wanted to use them as a guinea pig since. Frisk is immune to disease and is the only healthy human on Earth.