While Rarity and Fluttershy were busy trying to subdue Rainbow Dash at the surface, Twilight was left alone. She tried to convince Trixie, Lavender and Fushia to come with her to the shore, stay put and look for a solution. She didn't count on them actually liking being mermaids, for different reasons. And having a numerical advantage of three to one, they quickly overpowered Twilight and tied her up with seaweed, right before escaping away.

The TL;DR :
>Be Princess Twilight Sparkle

>Take Fluttershy and Rarity to confront Rainbow Dash, Trixie, Fuchsia Blush and Lavender Lace

>All of them mermaids, like yourself

>Fluttershy and Rarity try to subdue Rainbow Dash

>Leaving you vulnerable and in a three vs one disadvantage against Trixie and The Illusions

>End up becoming a living Sushi

Enjoy!