-The short is set 8 years after the end of the second movie, Sheryl has not awakened yet and Alto is still missing (I've read conflicting reports on the actual time so it may not be 8 years but it is some time after the second movie)
-Ranka is on a Interstellar tour
-New Protoculture ruins are discovered called "The Well of Souls" and Ranka goes there
-The ruins react to Ranka's song and she feels Alto's presence and jumps into the ruins
-Inside the ruins show many memories of Ranka (apparently all the memories are new scenes)
-Sheryl appears there although not physically obviously (Probably thanks to her earring?)
-Both sing in duet a new song that supposedly is really good
-After the song ends a beam of fold energy shoots and shows Ranka the exact location of Alto
So in summary:
Joking aside, the people that have watched it say that the new songs are really good and the animation is excellent, also all the characters appear, Mikhail, Klan, Ozma, Luca, Nanase and even Brera (just in the memories though).
But yeah it really didn't add much new and just maintained the Status Quo instead of advancing the story even if a little bit, personally I still prefer the new end credits to the Blu-ray box version where it heavily implies that the end credits concert is after Sheryl woke up and that the blue shooting star is Alto coming back.
In the end more Frontier is always welcomed but come on just let Alto and Sheryl reunite already!