Celestia has tired of seeing her sister, Luna, drink herself stupid day after day. Celestia can see it destroying Luna's life, but her attempts at intervention have failed. But now, she's given up convincing the alicorn to give up her moonshine, and instead is going to force her to cease in her destructive ways. Even if it means that she has to join in her sister's drunkenness to show her the kinds of problems it creates.

But why did Luna become an alcoholic upon returning from the moon? What was it that changed the once strong-willed, independent alicorn into a drunken mess?