This takes place four years before the main events. Kamito is assigned a mission from the Instructional School: steal two militarized Demon King / powerful spirits from the imperial capital during the Great Festival of the Spirits. At the festival, there is a grand procession with a portable shrine carrying the Princess Maiden of Flame. The shrine is guarded at its four corners by militarized spirits, two of which are what Kamito has to target. In the end, he only retrieves one of the two, because the Princess Maiden of Flame (Rubia) manages to intervene.
After the battle with Jio, Kamito falls ill due to exhaustion / overexertion. Est, left behind (because she couldn’t go with him / didn’t fight that time), takes care of him. She goes out to collect ingredients for a remedy (involving a family folk remedy from Laurenfrost) including scallions. (Some parts are trimmed/skipped in the adaptation)
Kamito, Est, Claire, and Fianna take a part‑time job at a maid café. It’s a lighter, comedic extra, emphasizing slice‑of‑life interactions rather than major plot events.
Claire and Rinslet go searching for a legendary “bust spirit” in a lake. (“Kyonyū” literally “large bust / large chest,” so it's kind of a fan‑servicy or comedic premise.) They try to find this spirit and interact with it.
Team Scarlet (Kamito + his contracted spirit + his companions) try a spirit summoning via a hot pot (“yaminabe” = literally “dark hot pot” / a kind of potlocked mixture) setup. Probably goes wrong / comedic battle ensues.
Kamito gets a rest / downtime in a hot spring. As usually happens with these kinds of OVAs, relaxation doesn’t last: the rest of the team walks in and things get awkward / comedic (fan service, misunderstandings, etc.).