At the invitation of the demon princess Emilia, Mary and her friends go on vacation to the demon homeland, the Dark Isle. But rather than the evil hellscape of fire and brimstone Mary was expecting, she's greeted by a southern paradise of sandy white shores and sparkling blue seas! Although Mary is excited for her first vacation abroad in both her lifetimes, fickle fate has other plans for her time off. How will the Invincible Little Lady unravel all the political machinations that threaten her and her friends' seaside relaxation?!