Once upon a time, the demon lord Rchimedes spread terror throughout the world, until he was eventually sealed away by the legendary hero Creasion. Since then, a thousand years have passed peacefully. However, a mysterious hole has opened up between the demon and human spheres, and countless demons have surged into the human realm once more. Coming to the conclusion that Rchimedes would soon return to wreak havoc, a human king summons the possible descendants of the legendary hero—all 75 of them. Unfortunately, after so long, it was too difficult to pinpoint his true descendants. Among the lionhearted prospects is the amateur adventurer Alba Frühling. His skills may not be top-notch, but he is accompanied by the talented soldier Ross, who helps the young hero whenever he is in a pinch...or at least, he is supposed to. Though undoubtedly a skilled warrior, Ross is actually both sarcastic and sadistic, and hence revels in Alba's suffering. Senyuu. is a comedic adventure following the unlikely duo as they struggle in their endeavor to defeat the demon lord, meeting various eccentrics along the way.

When a hole opened up in the world and monsters flooded out, humanity prepared for the demon lord Rukimedes' rise once again. However, even the gravity of this situation cannot impede the antics of straight man hero Alba Frühling and sadist soldier Ross. Ross, trapped within a plane between two worlds due to Januar Ein's magic, encounters the demon lord taking a nap and looking far too comfortable in his confinement. Simultaneously, Alba, once again in prison, has been sidelined in favor of his eccentric companion's hijinks. Although separated, Ross and Alba continue their attempts to prevent the demon lord's resurgence, but unbeknownst to them, a ghost from Ross' past may just threaten their quest's success.

Mahiru Shirota firmly believes that simple is best and troublesome things should be avoided at all costs. It is troublesome to do nothing and regret it later—and this ideology has led the 15-year-old to pick up a stray cat on his way home from school. As he affectionately names the feline Kuro, little does he know that this chance meeting will spark an extraordinary change in his everyday life. One day, Mahiru returns home to find something quite strange: a mysterious young man he has never seen before. His subsequent panic results in the uninvited guest being exposed to sunlight and—much to Mahiru's shock—transforming into Kuro! Upon revealing himself as a mere lazy shut-in vampire, Kuro promises to leave once night falls. However, one disaster after another leads to Mahiru accidentally forming a contract with his new freeloader, dragging him into a life-threatening battle of supernatural servants and bloodthirsty beings that is anything but simple.

Side stories released exclusively on DVD.