I Want to Save a Country, but It Was a Hundred Thousand Years Ago

"Your name is Nannan, right?" "That's right." "You may not believe what I say next. I come from one hundred thousand years in the future." "?" "The human race will perish in a hundred thousand years." "?!" "I will teach you the skills on the condition that you bury what you have learned throughout your life in a place after your death." "Okay." … Su Yu traveled through time and became the last human being in the world. Looking at the world full of ruins and devastation, he thought he would die soon. Until he had a dream, in which he came to a hundred thousand years ago... This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.