Twilight wasted no time getting to Canterlot upon being summoned by Princess Celestia. She'd received a scroll from the sun-raising princess earlier that day, asking her to come to her castle and see her at once about an urgent matter that involved her and her friends. What it was, the princess did not specify, though Twilight knew she'd find out soon enough.
As she sped into the huge castle and before the thrones of Princess Luna and Celestia, it was only Celestia that was present, with Luna likely asleep, due to her nightly duties.
Twilight stood patiently and waited for Celestia to finish talking to one of her famed Royal Guards before turning to her ex-pupil and bidding a greeting. "I've called you here because there is something very important that you must know. It could potentially be a very dangerous situation," Celestia began, stepping down from the throne and walking down beside Twilight and beyond, prompting the Friendship princess to follow.
Twilight took a deep breath. Dangerous? "I see. What is it? Is it Chrysalis? A disease? Some ponies that need friendship lessons?"
"None of those," Celestia replied flatly. "I'm afraid there is a changeling among your friends. They have been disguised as one for quite a while now, going completely unnoticed."
"There's a what?!" Twilight stopped in her tracks. A changeling? This can't be good! "So, how long has this been?"
They turned a corner, and a passing guard saluted before carrying on. "I've discussed this with Shining Armor and Princess Cadance, and we've agreed that it's possible the changeling has been around since the wedding."
Twilight gulped. "Geez... What do we do?"
"It's what you will do, Princess Twilight. You must find out which of your friends is not a friend at all. The changeling must be apprehended as soon as possible. Who knows what they're planning? We cannot wait any longer. It may even be too late. So you must go now and figure out which of your friends is fake."
Twilight nodded. "Of course. And I bring them to you?" She prepared to fly off on this "assignment" of sorts.
"Yes. Princess Cadance and Shining Armor are on their way as we speak. Capture the changeling and bring it here, and we will deal with it."
Twilight nodded once again. "Right away, princess. Though capturing might not be an easy task..."
"You, of all ponies can do this. I have utmost faith in you, my former student. Now go, before it's too late."
Twilight spread her wings and flew off, heading for Ponyville.
Her castle was empty when Twilight trotted through the double door entrance in search for Spike. Her number one assistant was sure to want in on this dilemma, and she'd certainly need his help, as well.
Going up the stairs to her room, she discovered the young dragon asleep on her bed, facedown in the pillow.
She gave a momentary smile before calling his name quietly. "Spike?"
He stirred, but didn't wake. So Twilight tried again, a bit louder.
"Spike! Wake up, I need your help."
He mumbled and rolled over, pulling the covers over him up to his neck and snuggling the pillow.
Twilight sighed and rolled her eyes, trotting over and pulling the covers off him. "Spike! Wake up!"
"Wah!" He leapt into the air, startled, and collapsed on the floor. He rubbed his head and grimaced, staring up at his friend. "Hey... What gives?"
"Spike, the Princess has put me on an important task, and I'll need your help to complete it," she replied, making the bed.
Spike stood up, still rubbing his head gingerly. "Well, what is it?"
Twilight fluffed the pillow and led Spike out of the room and down the stairs as she explained to him what Celestia had told her. As she expected, Spike was appalled.
"A changeling? All this time? But who?" He nibbled his claw-tips nervously as they reached the bottom of the stairs and headed for the front doors. "Do you have any ideas?"
"No, I don't. But that's what we're going to find out. Unfortunately, we'll have to gather all our friends to do it." She went outside, Spike on her tail, and went in the direction of Fluttershy's cottage, since she was closest.
"How are we going to do that?" Spike inquired, hopping onto her back for a ride.
She began trotting. "I have a few ideas..."
"I don't like the sound of this..."
Twilight went around some ponies walking slowly and talking. "Neither do I, but this has to be done. They could be up to something no good. They have to be up to no good. Celestia made a good point when she stated this could be potentially dangerous. If this changeling gets back to the Hive, it could have information to help Queen Chrysalis make a second attack! We can't let that happen."
Spike clung to her neck. "I'm just not looking forward to seeing it," he murmured.
"It's for the better, Spike. And as much as the others won't like this, they'll understand it has to be done. At least, the ones that aren't a changeling..." She made her way over the bridge to the animal-friendly cottage that was the Element of Kindness's home. "And I guess we'll just gather them all up and go from there."
"Wait, do what?"
Twilight reached the front door of the cottage and knocked firmly. "You'll see."
Spike swallowed hard. "I've got a real bad feeling about this..."