"Shingeki no Kyojin: Ilse no Techō - Aru Chōsa Heidan-in no Shuki" is a special OVA episode that serves as a side story within the larger narrative of the "Shingeki no Kyojin" series. The episode centers on a notebook discovered by Levi Ackerman and his squad during an expedition outside the walls. The notebook, written by Ilse Langnar, a former member of the Survey Corps, provides a harrowing account of her final moments after being separated from her unit and encountering a Titan.

Ilse Langnar’s story begins with her survival after a Titan attack leaves her stranded in Titan-infested territory. Alone and desperate, she stumbles upon a Titan that exhibits unusual behavior—it does not immediately attack her. Instead, the Titan appears to recognize her as a human and begins speaking in fragmented, incoherent phrases, addressing her as "Ymir-sama." This interaction suggests a rare instance of Titans retaining some form of human consciousness or memory, a concept that becomes significant in the broader series. Ilse’s shock and confusion escalate as she attempts to communicate with the Titan, but the encounter ends tragically when another Titan kills her.

The notebook, recovered by Levi’s squad, becomes a crucial piece of evidence for the Survey Corps, offering insights into the mysterious nature of Titans and hinting at deeper connections between Titans and humanity. The episode also highlights the psychological toll on soldiers who face the constant threat of Titans, as well as the lingering questions about the origins and behavior of these creatures. Ilse’s story, though brief, adds depth to the series’ exploration of human resilience and the enigmatic forces driving the conflict between humanity and Titans.