When retail worker Kosaka Haru sees a man being attacked on the street, he intervenes without hesitation. The next day, he wakes up in an abandoned building surrounded by a ravenous mob, realizing he has stumbled into a world he never knew existed. The man he saved, Akihito, reveals that Haru was mortally wounded in the attack and that the only way to save him was to bite him, turning him into a vampire. However, the transformation is not instantaneous. Haru now exists as a tank, a partially converted human whose blood is an irresistible and consequence-free meal for other vampires, as biting a tank does not create new offspring.

The story follows Haru as he navigates this dangerous in-between state while Akihito promises to protect him. Haru works in a clothing shop, and his ordinary life is now complicated by his gradual loss of humanity and the constant threat from other vampires who view him as prey. As Haru adjusts to his new circumstances, he finds himself developing feelings for Akihito, the vampire who gave him a second chance at life. The central conflict revolves around whether Haru will complete his transformation safely, whether his feelings for Akihito are genuine or merely a result of their sire bond, and what it truly means to give up his humanity.

The setting is contemporary Japan, though some sources place the story in the year 2096, a future where concepts like family responsibility have shifted and suburban areas have become poverty-stricken and crime-ridden. The narrative is contained within a single volume of seven chapters, with a complete story arc that focuses primarily on the relationship between Haru and Akihito rather than extensive world-building.

A notable narrative arc involves Haru encountering a suspicious priest who claims to have a way to reverse the transformation for tanks who do not wish to become vampires. This priest lures Haru to an abandoned church with promises of helping other unwillingly bitten humans, but his true intentions are far more predatory. This event forces Akihito to confront his feelings directly and serves as the climax of their relationship development. Another significant arc explores the possessive instincts Akihito experiences as Harus sire, the first person he has ever turned, and Harus uncertainty about whether Akihitos attachment to him is genuine love or merely vampire instinct. The story concludes with Haru accepting his new life and his feelings for Akihito, though the brevity of the series has led some readers to wish for more exploration of its unique vampire lore and character dynamics.