Under-tickles: A tickletale inspired story

Long ago two races ruled over Earth: Humans and Monsters.
One day, war broke out between the two races.
After a long battle, the humans were victorious.
They sealed the monsters underground with a magic spell.
Many, Many years later...
Plants and Zombies now rule over Earth.
They also went to war but it ended in peace between the two races.
Now... About 50 years after the humans vs monsters war, A Peashooter And Sunflower couple went on a hike together and found a cave in the mountain...

This story will be if Frisk couldn't open the barrier to the surface. Also, Frisk is female, 15 and fell into the underground by accident while trying to escape the zombies. with that said, let's start the adventure of Peater and Sunny through the underground.