You're a human, and after a failed exploration, you fell into the Underground, where people like Sans, Papyrus, Grillby, Asgore, Undyne, Alphys, etc. live. Cursed by the humans and bound to the Underground, they do what they can to make it their home, building houses, puzzles, libraries, schools... everything a normal town would have. They have six human souls, and they need seven to cross the barrier and shatter it for good... YOUR soul.