Rainbow and Twilight have both had crushes on each other for a while. When Twilight accidentally breaks Rainbow's heart, Rainbow runs straight into the Everfree forest, endangering both her and her precious crush
"Looks like somepony has a crush on the new guy!" "WHAT! No, no I don't!" "Oh, she does! She totally does!" You heard them. Twilight has a crush on somepony else. You'll never have a chance with her. Just give it up, Dash.
After an...accident causes Rainbow Dash to lose most of her body, it being replaced with robotic parts and artificial intelligence, Rainbow and her friends must get used to her new life.