Rainbow Dash jumps out of her cloud bed as a huge bang erupts under her cloud house. "What the hay was that!" She fly's out the front door and looks down to see Pinkie firing her party cannon straight at her cloud house. Dash lands next to Pinkie and grabs her. "What are you doing!"
Pinkie gets a worried look. "I just heard from Twilight. Apparently she put up a spell to detect when a changeling changes form. She says its been there since the attack on Canterlot. Dash, SHE KNOWS THERE'S A CHANGELING IN PONYVILLE."
Dash shoves a hoof in Pinkies mouth. "Quiet pinkie."
Pinkie pushes Dash's hoof away. "But Dash, she plans on using that reveal spell she used in Canterlot to discover who it is. If she uses it you'll be exposed and nopony will believe you when you try to tell them the truth because you've been hiding this secret so long and you didn't reveal yourself so Twilight had to force you to." The pink pony's face had turned blue and she starts gasping for breath. The realization of what Pinkie slowly dawns on Rainbow Dash.
Rainbow Dash thanks Pinkie for telling her about Twilight's plan and fly's off towards the library. As soon as she arrives she bolts inside and finds Twilight concentrating, her horn glowing. Fear floods her body as she tackles Twilight "NO DON'T DO IT YET."
Twilight jumps up. "Whats wrong with you Dash!!"
Dash runs up and grabs Twilight in a hug. "Don't use the reveal spell to find the changeling. Please just don't."
Twilight looks at Dash like shes crazy. "What are you talking about? That was a locating spell to find Spike. I havent figured out how to make the reveal spell spread throughout Ponyville yet. That's why I need him."
A look of relief passes Dash's face. "Phew. That's a relief."
Twilight does a double take of sorts. "Wait, why don't you want me to reveal the changeling?"
Just then Pinkie runs into the room. 'Thank Celestia for Pinkies perfect timing.' Dash sighs.
Pinkie walks up to Twilight. "You can't use it yet because me and Dash think we know who it is, but we want to try to figure out what they want before we get rid of them. Besides, if they've taken the place of somepony here, shouldn't we get them to spill the beans on where their hiding the pony before we chase them out of town. If they had kidnapped a pony to take their place and hidden them where only they know where to look, We could lose them forever."
Twilight thinks about this for a moment "Your right Pinkie. We don't want to lose a friend forever just because i was hasty in casting a spell."
Dash and Pinkie breath a sigh of relief. Dash walks over and kisses Twilight. "Thank you Twilight. I promise well get to the bottom of this fast."
"You had better, because if somepony gets hurt i'm casting the spell." Twilight kisses back. "Now go and find the changeling."
Dash and Pinkie run out of the library and back to Sugar Cube Corner. When they arrive, Dash manages to pant "Ok, now what."
Pinkie just shakes her head. "You have to tell her Dash. There's no other way.
Dash shakes her head. "No, i just cant do that. But i do have a plan. I need you to gather up the other three and bring them back to the library. Don't tell them anything, just that Twilight knows their's a changeling in Ponyville. Ill explain when i get there." She turns to leave
Pinkie grabs her and spins her around. "Where are you going?"
Dash gives her a huge hug. "I'm going to pack, just in-case. Ill see you there Pinkie." Dash walks outside and jumps into the air to head back to her cloud home. Once she arrives she packs her gala dress and her few belongings that she left when she moved into the library with Twilight. Just thinking about her mare friend causes Dash to break out in tears again. She strokes a picture of her "I'm going to miss you so much. I hope you will accept me for who i am, otherwise I don't know how ill go on living without you."
A few hours later, Rainbow Dash walks into the library after having packed everything. When she enters she's peppered with questions from the others (excluding Pinkie of course). She holds up one hoof. "Twilight, is the spell ready?" Twilight just nods. "Ok, i have the answer you seek, but before you hear it, everypony form a line and face Twilight." As soon as they were all facing Twilight Dash continues. "I have discovered the identity of the changeling, however before i tell you i want you to know that they haven't replaced any of our friends. In fact, they are one of our friends. They have been living as a pony since they were a foal. So i want everypony to think about this. If we use this spell we will be removing a great friend from our life forever. Do you really want to do that?"
Twilight looks just as shocked as everypony else, but she snaps out of it. "I don't want to do it, but i believe it has to be done.
Dash looks around at everypony nodding their head. "In that case, the changeling is standing in this very room." All heads turn to look down the line, wondering who it could be. All the heads besides Pinkie's and Dash's, who just reach over and hug each other. They both say "goodbye". When the other four see this they realize it is eater Dash or Pinkie. They all take a step away from the duo as Dash ends the hug. "Ok Twilight, were ready."
Twilight nods." The spell will take a few moments to take effect after i finish casting it."
Dash and Pinkie nod. Twilight's horn lights up and flashes, then goes dark. Twilight closes her eyes, as does everypony there except Rainbow Dash and Pinkie. Dash feels a tingling running through her body and looks at her hooves, their covered in green sparks, a byproduct of her changeling magic trying to battle Twilight's. Making one last move as Rainbow Dash, she rush's towards Twilight, who still has her eyes closed, and grabs her in a hug while kissing her. Twilight opens her eyes just enough to see Dash's eyes turn green, then her whole body changes in an instant. "I love you Twilight Sparkle."