"Yubiwa no Eranda Kon'yakusha" is a fantasy romance manga that centers on a young woman named Lilianne, who becomes entangled in a supernatural contract after discovering a mysterious ring. The ring, which she finds in her family's possession, binds her to a demon named Zagan, who claims her as his bride. The story explores the dynamics of their relationship as Lilianne navigates her new life with Zagan, who is both protective and possessive, while also uncovering the secrets of the demon world and the true nature of the ring.

Lilianne is portrayed as a kind-hearted and determined individual, often struggling to reconcile her human values with the harsh realities of the demon realm. Zagan, on the other hand, is a powerful and enigmatic figure whose motives are initially unclear. As the narrative progresses, their bond deepens, and Lilianne begins to understand the complexities of Zagan's past and the responsibilities he carries as a high-ranking demon. The story delves into themes of trust, loyalty, and the blurred lines between love and obligation, as Lilianne and Zagan face external threats from other demons and humans who seek to exploit their connection.

The manga also introduces a supporting cast of characters, including other demons and humans who play significant roles in the unfolding events. These characters often serve to highlight the cultural and ideological differences between the human and demon worlds, adding depth to the narrative. The plot is driven by a mix of political intrigue, supernatural battles, and emotional conflicts, as Lilianne and Zagan work together to protect their bond and uncover the truth behind the ring's origins. The story balances moments of tension and action with quieter, introspective scenes that explore the evolving relationship between the two protagonists.