You have been wandering around the hole to Underground, from mountain ebbot. You thought you wont be this stupid, to just fall into it like other humans, but wrong.

By accident, your foot slipped on brake, and fell down into the hole... You woke up from unconsciousness, after hearing a voice...

Hello, is anyone there..? She soon noticed you Oh.. you must've fallen down here.. come with me, My name is Toriel, the caretaker of the Ruins. I will lead you somewhere safe in there..