A scourge has descended upon vampire society in the form of serial murders. The killings appear unrelated at first glance, but every crime scene has one thing in common: a card left behind, bearing the words True of Vamp—the name given to the legendary progenitor of all vampires. The Vlad Agency, a secret intelligence operation, summons the renowned aristocrat Dali Delico to lead the task force investigating the murders. However, Dali refuses, choosing to focus on raising his children. His old colleagues—Gerhard Fra, Henrique Lorca, and Dino Classico—plead with him to balance his duty to the agency with child-rearing. He agrees on one condition: the other three men must also care for their children during the investigation. Dali's estate therefore becomes a nursery and the base of operations for the four noble vampires. Meanwhile, a criminal organization Dali knows all too well lurks in the shadows with evil intentions that threaten to cause the collapse of the entire vampire society.

Reeling from her recent breakup, Momo Ayase, a popular high schooler, shows kindness to her socially awkward schoolmate, Ken Takakura, by standing up to his bullies. Ken misunderstands her intentions, believing he has made a new friend who shares his obsession with aliens and UFOs. However, Momo's own eccentric occult beliefs lie in the supernatural realm; she thinks aliens do not exist. A rivalry quickly brews as each becomes determined to prove the other wrong. Despite their initial clash over their opposing beliefs, Momo and Ken form an unexpected but intimate friendship, a bond forged in a series of supernatural battles and bizarre encounters with urban legends and paranormal entities. As both develop unique superhuman abilities, they learn to supplement each other's weaknesses, leading them to wonder if their newfound partnership may be about more than just survival.

One night, as meteors streak across the star-studded sky, Shion Pavlichenko becomes a Contractor. Despite her brother's transformation, Shion's twin sister Suou continues to live a fairly ordinary life, attending middle school with her friends and getting caught up in the awkwardness of growing up. However, everything changes when her home is invaded by a masked man cloaked in black, destroying any sense of normality she once had. Revealed to possess latent Contractor abilities of her own, Suou is caught between family, friends, and her own sense of purpose as she ventures into the ruthless world of cutthroats and espionage that Contractors call home. Meanwhile in Tokyo, investigation surrounding Hell's Gate's sudden collapse is underway, and prophetic signs of doom point in the direction of a silver-haired doll.