You fall to Underground.

Flowey: Howdy, I am Flowey the flower. I shall teach you how things work here. Ready? Here we go!

You enter a FIGHT and are a red heart.

Flowey: See this heart? This is you. You start off weak but can grow strong if you gain many LV. What is LV stand for? LOVE of course. Get these friendliness petals, it will give you LV.

You move in them as Flowey shoots them. It deals you damage, you have 1 HP out of 20.

Flowey: You idiot, in this world is kill or be killed.

Toriel saves you shooting Flowey with a fireball.

Flowey: Ahhh!

Toriel: Come, my child.