Determined Integrity. (English translation)

Manny (11) and Frisk (10) are two energetic and imaginative children, to the point of standing out from the crowd compared to the other children in town. Unfortunately, the fact that they are different causes bullying, so the two friends have decided to skip class and go explore Mount Ebott instead...
However, a mixture of too much curiosity and a bit of clumsiness causes them to fall into the hole. So the two kids have to explore the Underground in search of an exit, and maybe make a few friends along the way - the ghost says monsters are nice after all.