"Attack on Titan: Lost Girls" is a spin-off of the main "Attack on Titan" series, focusing on two side stories that delve into the perspectives of Annie Leonhart and Mikasa Ackerman. The narrative is divided into two parts: "Wall Sina, Goodbye" and "Lost in the Cruel World," each exploring the inner struggles and backstories of these characters.

"Wall Sina, Goodbye" centers on Annie Leonhart, a member of the Military Police Brigade. The story begins with Annie being tasked with locating a missing noblewoman within Wall Sina. As she investigates, the narrative shifts between her current mission and flashbacks to her childhood, revealing her complex relationship with her father and the harsh training she endured. These flashbacks provide insight into her motivations and the emotional burdens she carries. The mission itself is fraught with tension, as Annie navigates the political intrigue and moral ambiguities of her role, ultimately leading to a confrontation that tests her resolve and humanity.

"Lost in the Cruel World" shifts focus to Mikasa Ackerman, offering an alternate reality where her parents are still alive, and she lives a peaceful life. However, this idyllic existence is disrupted when Eren Yeager, her childhood friend, goes missing. Mikasa embarks on a journey to find him, encountering various challenges that force her to confront her fears and insecurities. This story explores Mikasa's deep bond with Eren and her internal conflict between her desire for a normal life and her unwavering loyalty to him. The alternate setting allows for a deeper examination of Mikasa's character, highlighting her strength and vulnerability.

Both stories are character-driven, providing a more intimate look at Annie and Mikasa outside the main series' larger conflicts. The narratives are tightly woven, blending action, introspection, and emotional depth. The themes of identity, duty, and personal sacrifice are prevalent throughout, offering a nuanced exploration of these characters' psyches. The spin-off enriches the "Attack on Titan" universe by expanding on the backstories and motivations of two of its central figures, adding layers to their roles in the overarching narrative.