Illya and her friends head off to a hot springs inn, where Rin and Luvia turn out to be working on a secret mission of their own. While the group is there, Kuro realizes the hot springs are draining her mana far faster than normal, and the cause is a mystery. The OVA is a single magical girl episode that mixes action, comedy, fantasy and ecchi. It adapts Hiroyama Hiroshi's manga, a spin-off of Type-Moon's Fate/stay night visual novel, and was packaged with the limited edition of the sixth volume of the Prisma Illya 3rei!! manga.