In the year 2020, a nation is under constant assault from the "Others," mysterious creatures that descend from the sky and prey on human minds. Around the same time, scientists identify a hormone called psionics, which grants certain individuals remarkable mental abilities useful for battling these monsters. The country's leader establishes the Other Suppression Force (OSF), a military unit tasked with protecting society from the Others. Its defenders—known as Scarlet Guardians—are young people blessed with psionic powers, rigorously trained to join the OSF.
One day, the nation holds a ceremony honoring outstanding trainees who have completed their preparation to become full-fledged Scarlet Guardians. Among them is Yuito Sumeragi, the dedicated child of the nation's leader. Saved by an OSF woman when he was five years old, Yuito joins the force to repay that debt, ready to put his training to use. As he gets to know his new teammates, he notices a startling resemblance between one of them and the woman who rescued him years ago. Yet this discovery is minor compared to the looming conspiracy that threatens the entire country—a secret hidden from Yuito's awareness.