"Pupa" is a psychological horror anime that explores themes of transformation, survival, and the bond between siblings. The story centers on Utsutsu and Yume Hasegawa, a brother and sister living in a dysfunctional household. Their lives take a dark turn when Yume becomes infected with a mysterious virus known as "Pupa," which causes her to transform into a monstrous creature with an insatiable hunger for human flesh. Despite the grotesque nature of her condition, Utsutsu remains fiercely protective of his sister, determined to keep her safe and find a way to cure her.

The virus grants Yume regenerative abilities, making her nearly immortal, but it also amplifies her violent impulses. Utsutsu, who harbors deep emotional scars from their abusive upbringing, willingly offers his own flesh to satisfy Yume's hunger, showcasing the extreme lengths he will go to for her survival. Their relationship is fraught with tension, as Yume struggles to control her monstrous urges while Utsutsu grapples with his own guilt and trauma.

As the siblings navigate their grim reality, they encounter other individuals affected by the Pupa virus, including scientists and government agents who seek to study or exploit the phenomenon. The narrative delves into the origins of the virus and its connection to a secretive research facility, revealing a broader conspiracy that intertwines with the siblings' personal struggles. The series is marked by its intense and often graphic depictions of body horror, emphasizing the physical and psychological toll of the virus on both Yume and Utsutsu.

The anime's pacing is brisk, with a focus on the emotional and moral dilemmas faced by the characters rather than extensive world-building. The dynamic between Utsutsu and Yume serves as the emotional core of the story, highlighting themes of sacrifice, loyalty, and the blurred line between love and obsession. Despite its short runtime, "Pupa" leaves a lasting impression through its unsettling atmosphere and exploration of the darker aspects of human relationships.