Akari finds herself nervous coaching a new undine as a Prima, until she awakens to find that it is just a dream. To soothe her worries, Alicia recalls stories of her beginnings as a Prima, as well as the memories she made.
"Aria Arietta" is the OVA of the second season of aria, "Aria the natural".
The entire chronology is:
"Aria the animation", "Aria the natural", this OVA and "Aria the origination"
I made a mistake and watched this after the third season, it spoils it a little bit ;)
"Aria arietta" is just like any other episode of aria, although I would count it among the better.
It has a fitting story, good music and seems like a good step toward the next season.
I hate sounding like a broken record, but aria is great for relaxing and forgetting the worries of your busy life.