This story is a sequel to The Best of All Possible Worlds
For generations, humanity has dreamed of discovering intelligent life out in the stars. To say that ending that search in downtown Pasadena was a surprise would be an understatement.
Princess Luna had herself a simple mission: Track Twilight Sparkle and her friends through a malfunctioning portal to Earth, and get them out without being noticed. Luna had no desire to establish permanent relations, or even to stay and talk. After all, both princesses were convinced from previous experience that humans were nothing but trouble.
It appears that both parties have much to learn about each other.
Present-day follow-up to "The Best of All Possible Worlds". "Alternate Universe" tag is because it pretty quickly diverges from Season Two continuity.