"Oniyome Diary" centers around the life of a young man named Takuma, who unexpectedly marries a demon girl named Yuuhi. The story explores their daily life as a newlywed couple, blending elements of comedy, romance, and supernatural themes. Yuuhi, despite her demonic origins, is kind-hearted and deeply devoted to Takuma, though her otherworldly nature often leads to humorous and chaotic situations. Takuma, an ordinary human, finds himself navigating the challenges of living with a demon wife, from her peculiar habits to her overwhelming strength and occasional misunderstandings of human customs.

The narrative unfolds through episodic chapters, each highlighting the couple's interactions with both human and supernatural characters. Yuuhi's demon family occasionally appears, adding layers of complexity to the story as they bring their own quirks and traditions into the mix. The series also delves into the cultural differences between humans and demons, often using these contrasts to create comedic or heartwarming moments.

As the story progresses, the bond between Takuma and Yuuhi deepens, showcasing their growth as a couple. While the tone remains lighthearted, there are moments of genuine emotion as they face challenges together, such as societal prejudices against their union or misunderstandings stemming from their differing backgrounds. The manga balances its comedic elements with a sincere exploration of love, acceptance, and the blending of two worlds. Through its charming characters and engaging premise, "Oniyome Diary" offers a unique take on the slice-of-life genre, infused with supernatural flair.