A Cozy Glow-hating brony is fooled by the nefarious filly herself into switching bodies, now he's the one locked in Tartarus while she's causing havoc in his world. Uh-oh. Also, nopony believes him... because he's Cozy Glow. Makes perfect sense.
Our hero finds herself in a strange world, and in a strange new form. Maybe this could be a fresh start, you ask? The thing is that she wasn't aware she needed one.