"Itadaki! Seieki" is a manga series that blends supernatural and romantic elements within a comedic framework. The narrative centers around a young man named Aoi, who encounters a succubus named Hikari. Hikari, unlike traditional succubi, is not malevolent but rather struggles with her role as a demon due to her kind-hearted nature. She feeds on the life energy of humans, but her method of doing so is non-lethal and often leaves her victims unharmed, albeit exhausted.

The story begins with Hoi accidentally summoning Hikari during a ritual he performs out of curiosity. From this point, the two form an unusual bond. Hikari moves into Aoi's home, and their relationship evolves as they navigate the challenges of cohabitation. Hikari's presence introduces a series of comedic and supernatural events, often stemming from her attempts to adapt to human life and her duties as a succubus. Aoi, initially overwhelmed by the situation, gradually becomes more accepting of Hikari and her quirks.

The plot explores the dynamics of their relationship, with Hikari's attempts to seduce Aoi for his life energy serving as a recurring theme. However, these attempts are often thwarted by her own ineptitude or Aoi's resistance, leading to humorous situations. Over time, their interactions reveal deeper emotional connections, as Hikari begins to question her purpose and Aoi becomes more protective of her.

Secondary characters, such as other supernatural beings and humans, occasionally appear, adding complexity to the story. These characters often serve to highlight the contrast between Hikari's gentle nature and the more traditional, predatory behavior of other demons. The series balances lighthearted moments with occasional introspection, particularly regarding Hikari's identity and her place in both the human and demon worlds.

The narrative maintains a focus on the evolving relationship between Aoi and Hikari, blending humor, romance, and supernatural elements. The series does not delve deeply into overarching conflicts or high-stakes drama, instead prioritizing character-driven interactions and situational comedy. The tone remains consistent throughout, with a mix of playful and heartfelt moments that define the central relationship.