Frisk left the underground ,leaving monsters in sadness.

In 202X,Frisk decided to visit the underground again to paint the sins back.

After they fell to the underground again,they find something different.

Flowey warned them not to proceed,but Frisk insists.They find Toriel,telling Frisk the truth:the whole underground is spreading a serious disease which can kill the monsters and their souls.They will lose their magic at first,then walk towards death...

After Frisk leave the ruins,they got a diary from Toriel,which recorded she trying to find a way to heal the monsters,however,she has caught the disease as well.

Frisk met Sans and followed him to the Container--the base where monsters treat and heal monsters.Then Frisk find that Papyrus was in his last breath.

Holding Sans' hands,they watched Papyrus fade away.

Few days later,Sans took Frisk to the capital and meet queen Toriel again.Toriel was dying,and Sans didn't want to see more people die,so he took Frisk to the True Lab to find Alphys,to proceed the Determination Project...

(to be continued)