"Strike the Blood: Append" is a spin-off manga series that expands on the events and characters of the original "Strike the Blood" light novel and anime series. The narrative is set in Itogami Island, a man-made island inhabited by humans and supernatural beings, where the protagonist, Kojou Akatsuki, inadvertently becomes the Fourth Primogenitor, the world’s most powerful vampire. His newfound powers attract the attention of various factions, including the Lion King Organization, which assigns Yukina Himeragi, a Sword Shaman, to monitor and potentially eliminate him if he becomes a threat.

The story in "Append" focuses on episodic adventures that delve deeper into the relationships and dynamics between Kojou and the supporting cast, including Yukina, as well as other key characters like Asagi Aiba, a tech-savvy girl with a crush on Kojou, and Nagisa Akatsuki, Kojou’s adoptive sister. These stories often involve supernatural threats, political intrigue, and the consequences of Kojou’s powers, while also exploring the emotional and personal struggles of the characters. The narrative maintains a balance between action-packed sequences and moments of character development, providing additional context and depth to the main series.

Themes of loyalty, identity, and the coexistence of humans and supernatural beings are central to the plot. The series frequently highlights the moral dilemmas faced by Kojou as he navigates his role as a Primogenitor while protecting those he cares about. The episodic nature of "Append" allows for self-contained stories that contribute to the overarching narrative, offering fans of the series a deeper understanding of the world and its inhabitants. The manga retains the core elements of the original series while introducing new challenges and adversaries, ensuring continuity and engagement for readers familiar with the franchise.