"Shin Megami Tensei: If..." is a manga adaptation of the 1994 role-playing video game of the same name, set within the larger "Shin Megami Tensei" universe. The story unfolds in a high school setting, where the protagonist, a student named Hibiki Kuze, and his classmates find themselves trapped in a demon-infested alternate dimension known as the Expanse. This dimension is a twisted reflection of their school, created by the malevolent entity known as Nemissa, who seeks to corrupt and destroy the human world.

The narrative begins with Hibiki and his friends encountering a mysterious transfer student named Nemissa, who quickly reveals her true nature as a demon. She orchestrates a ritual that transports the students into the Expanse, where they must confront their inner fears and desires, manifested as demons. Hibiki, along with his childhood friend Aya Nishitani and classmate Charlie, must navigate this perilous world, battling demons and uncovering the truth behind Nemissa's motives.

As the story progresses, Hibiki discovers that he possesses the ability to summon and control demons, a power granted by a mysterious entity known as the Demon Summoning Program. This ability becomes crucial as he and his companions face increasingly powerful foes, including other students who have succumbed to the Expanse's corrupting influence. The group's journey is marked by moral dilemmas and choices that test their resolve, as they must decide whether to fight for their survival or succumb to the darkness.

The climax of the story involves a confrontation with Nemissa, who reveals her tragic backstory and her desire to escape her own suffering by destroying the human world. Hibiki and his friends must unite to defeat her and restore balance to both the Expanse and their own world. The resolution of the story hinges on the choices made by the characters, emphasizing themes of redemption, sacrifice, and the consequences of one's actions.

The manga adaptation retains the dark, atmospheric tone of the original game, with a focus on character development and psychological depth. The artwork effectively captures the eerie and oppressive atmosphere of the Expanse, while the narrative explores the complexities of human nature and the struggle between good and evil. The main characters, particularly Hibiki and Aya, are portrayed with a sense of realism and vulnerability, making their journey through the Expanse both compelling and emotionally resonant.