This is based off the comics, showing Marceline's past.
If you can't read the text, here it is:
1) "Whoa, check this place out!! And it even comes with a cool creepy bag! I could TOTALLY stay here until it's safe outside again. Actually, I don't--I don't think I like this. It feels weird. Its kinda burning my hand. Maybe I shouldn't be here after all."
2) "Marcy, I know the mushroom bombs have...changed things. Awakened...things. And I know I've been alone for so long that I've started talking to myself. But don't be afraid, Marceline! We're tough! We're smart! We'll make it through this. I promise."
3) "It's fine. There'll be somewhere else to stay. Somewhere else where we can find...friends.
Besides, it can't stay like this forever, right?... Everything is gonna be fine. It'll just take some time."
Marceline (c) Adventure Time
Mushroom Wars (c) Adventure Time