Twilight Sparkle worries that she might one day grow bitter and spiteful, and turn into somepony akin to Nightmare Moon. To combat this, Twilight prepares a spell that will let her go inside her mind and battle her negative personality traits while they're still weak and unformed. It doesn't end well.

Rarity and Spike eventually reconstruct the notes that Twilight used to create her spell, and decide to go inside Twilight's mind to wake her from her magically-induced coma.

It also doesn't end well.

But a bad ending isn't going to stop Rarity and Spike from trying to rescue Twilight anyway.