After the changelings' failed invasion of Canterlot, Twilight tries to soothe some of Ponyville's rampant paranoia by casting a spell to prove there are no imposters hiding in the town. But her spell leaves her in an alien body, facing a mob of angry ponies who just don't believe her when she says she is Twilight Sparkle.
Or, at least, she was Twilight Sparkle, anyway...