After unwittingly drinking a love potion Neptune falls madly in love with Nepgear and gets rather clingy towards her, meanwhile Nepgear struggles with her own feelings for Neptune. Soon enough, hidden feelings are brought to the surface as Neptune struggles to understand how she really feels about her beloved little sister, while said sister tries to lock away her own feelings.

After a little dispute with the other goddesses, Histoire orders Neptune do something for her nation for once. Neptune and her little sister go on a simple monster-slaying quest, but they didn't, in their pretty much immortal lives, expect to find something of great value.