Mahiru Shirota, an energetic 15-year-old, leads a simple life. However, that all changes, when he rescues a stray black cat he names Kuro. Seemingly an ordinary feline, Mahiru is taken aback when it transforms into a person! It turns out that Kuro is an outlandishly lazy and sluggish centuries-old vampire who does not drink blood and finds any kind of activity bothersome.

Unbeknownst to him, the one thing Mahiru should not do is say Kuro's name aloud, but he does exactly that. It forms a bond between the two, meaning Kuro could become Mahiru's servant vampire—a "servamp." But as fate would have it, after a vicious attack arranged by a vampire called Tsubaki, Kuro drinks Mahiru's blood, thus sealing the deal, making Mahiru his master—"Eve."

The mysterious vampire Tsubaki reveals Kuro's real name: Sleepy Ash, the sloth and oldest sibling of seven—each representing one of the deadly sins. Tsubaki, as the forgotten eighth brother representing "melancholy," wages war against all vampires and humans. To prevent bloodshed, Mahiru and Kuro now have to alert the rest of the servamps and their Eves before it is too late.