"Seiken Tsukai no World Break" is set in a world where certain individuals, known as Saviors, possess memories of past lives and wield supernatural abilities tied to those memories. The story centers on Moroha Haimura, a transfer student at Akane Academy, a school dedicated to training Saviors to combat dark forces known as Metaphysicals. Moroha is unique in that he retains memories from two distinct past lives: one as a powerful swordsman and another as a skilled magician. This duality grants him access to both physical combat techniques and magical abilities, making him an exceptionally versatile fighter.

At Akane Academy, Moroha reunites with two girls from his past lives: Satsuki Ranjou, a fiery and determined swordswoman who was his younger sister in one life, and Shizuno Urushibara, a calm and composed mage who was his lover in another. Both girls harbor strong feelings for Moroha, leading to a dynamic interplay of rivalry and camaraderie as they navigate their shared history and current challenges. The trio forms a close-knit team, working together to protect humanity from the Metaphysicals, monstrous entities that threaten the world.

The narrative unfolds through a series of episodic battles against Metaphysicals, interspersed with moments of character development and exploration of the protagonists' past lives. Moroha's dual abilities, known as "Pride" (physical combat) and "Kuroma" (magic), play a crucial role in their missions, often tipping the balance in their favor. As the story progresses, deeper mysteries surrounding the origins of Saviors and the true nature of Metaphysicals are gradually revealed, adding layers of intrigue to the plot.

The series balances action, fantasy, and elements of harem, with a focus on the relationships between Moroha, Satsuki, and Shizuno. Their interactions are shaped by their shared pasts, creating a blend of tension and affection that drives much of the character-driven drama. While the primary conflict revolves around the battle against Metaphysicals, the emotional core of the story lies in the characters' efforts to reconcile their past identities with their present lives, forging new bonds while honoring their shared history.