On an ordinary spring break evening, siblings Umehiko and Momo Ishikawa witness a mysterious symbol appear in the sky, which suddenly transports them from their world into a strange fantasy realm. Concerned only for his younger sister's safety, Umehiko manages to send Momo back through the magical seal that brought them there. However, he is pulled back at the last moment and left stranded in this new world alone.

Umehiko is immediately confronted by Theodorus Tramble, a powerful and arrogant sorcerer who has lived for five hundred years. Theodorus had been working tirelessly to summon a blessed maiden, a being whose body serves as a vessel of immense magical energy that a sorcerer can draw upon to restore and amplify their power. Upon realizing Umehiko is not the maiden he expected, Theodorus declares that the young man will become his source of unlimited power instead. He places a strange symbol, a heart-shaped insignia, on Umehiko's body. This enchantment grants Umehiko an infinite supply of magical power coursing through him, but it also comes with a severe drawback: the only way to release this magic is through physical contact with Theodorus, which triggers uncontrollable lust that only the sorcerer can satisfy. Umehiko thus becomes the sorcerer's blessed maiden, tasked with dispensing magic through his body.

The main characters include Umehiko Ishikawa, a protective older brother who finds himself trapped in a humiliating and physically demanding role. Theodorus Tramble is the high-ranking sorcerer who relies on obtaining fluids from his blessed maiden as his source of magic. Momo Ishikawa is Umehiko's younger sister, who was originally meant to be the blessed maiden but was sent home by her brother. It is later discovered that Momo has somehow returned to the fantasy realm as a slave. Rogii is a small monster, a yatil, who lives with Umehiko as a pet.

The narrative unfolds as Umehiko must endure his new existence as Theodorus's blessed maiden, struggling between his desire to escape and the physical bond that ties him to the arrogant sorcerer. The story explores the developing and complicated relationship between the two as they navigate this arrangement, while Umehiko also searches for a way to ensure his sister's safety after learning of her return to the realm. The manga, written and illustrated by Yuiko, is an otherworldly boys love fantasy that blends magic, coercion, and reluctant intimacy. The Japanese title is 前略、お兄ちゃんは聖女になりました。