"Scarlet Nexus" is set in a futuristic world where humanity faces constant threats from mysterious entities known as the Others. These grotesque creatures descend from the Extinction Belt, a region in the sky, and prey on humans, consuming their brains. To combat this existential threat, the Other Suppression Force (OSF) is established, employing individuals with supernatural abilities called psionics. These powers, ranging from telekinesis to pyrokinesis, are granted by the Brain Pseudo-Network, a system that connects human brains to a collective network.

The narrative unfolds through two parallel perspectives: Yuito Sumeragi and Kasane Randall. Yuito, a kind-hearted and idealistic recruit from a prestigious political family, joins the OSF to protect others and uncover the truth behind his father's disappearance. Kasane, a reserved and highly skilled soldier, is driven by a mysterious mission tied to her past and a cryptic vision of the future. Both protagonists are part of the OSF's Special Operations Unit, tasked with investigating the Others and the enigmatic organization known as Togetsu, which appears to have ties to the Extinction Belt.

As the story progresses, Yuito and Kasane uncover a web of conspiracies involving the OSF, Togetsu, and the origins of the Others. The Brain Pseudo-Network, initially seen as a tool for humanity's survival, is revealed to have darker implications, including the potential for mind control and manipulation. The protagonists also grapple with personal revelations, such as Kasane's connection to a girl named Naomi and Yuito's family secrets, which tie into the larger mystery of the Others and the Extinction Belt.

The series explores themes of identity, memory, and the ethical implications of technological advancement. Supporting characters, including members of the OSF and Togetsu, play crucial roles in unraveling the plot, each contributing unique abilities and perspectives. The narrative builds toward a climactic confrontation that challenges the protagonists' understanding of their world and their place within it, culminating in a resolution that addresses the immediate threat while leaving room for future exploration of the series' richly constructed universe.