Hininden: Gauss

All he wants is to tell his girlfriend he loves her — but this regular modern guy's night takes a sharp turn when the moonlight pulls him into an entirely different era, and suddenly Daisuke is tangled up in a historical fantasy world where a princess named Kurama is on the run from a powerful faction called the Rikudoushuu. Two shinobi, Kaede and Momoka, are trying to keep her alive long enough to summon a legendary entity called Gauss — and Daisuke, completely out of his depth, somehow becomes the vessel for that power. This single-episode OVA from Arms Studio leans into the isekai-before-isekai-was-everywhere setup, mixing feudal Japanese aesthetics with supernatural action and explicit content. The character designs by Rin Shin are detailed and distinctive, giving the whole thing a polished look for what it is. The story moves fast — one episode means no padding, just setup and payoff. If you've watched La Blue Girl or Demon Beast Invasion and appreciated how those titles actually tried to build a world around their content rather than just stringing scenes together, Gauss scratches a similar itch. It's not going to change your life, but for a 2005 hentai OVA, it puts genuine effort into its premise. The historical fantasy framing gives it more texture than you'd typically expect from the genre, and the chase sequence involving Princess Kurama has real tension before things shift in the expected direction.