"World Trigger" is set in Mikado City, a location frequently targeted by interdimensional beings known as Neighbors. These invaders pose a significant threat to humanity, prompting the establishment of the Border Defense Agency, an organization equipped with advanced weaponry called Triggers. These Triggers allow agents to combat Neighbors effectively by harnessing interdimensional energy. The story primarily revolves around Osamu Mikumo, a low-ranking Border agent with limited combat skills but a strong sense of strategy and determination. His life changes when he encounters Yūma Kuga, a humanoid Neighbor with mysterious origins and exceptional combat abilities. Yūma joins Border under Osamu’s guidance, and the two form a partnership to protect the city.

The narrative explores the dynamics of Border’s internal structure, which is divided into squads that compete for rank and resources. Osamu and Yūma, along with their ally Chika Amatori, a young girl with immense Trion energy but no combat experience, form a team to rise through the ranks. Chika’s unique abilities make her a target for both Neighbors and rival factions within Border, adding tension to their journey. The series delves into the complexities of teamwork, strategy, and personal growth as the trio faces increasingly challenging missions, including large-scale invasions by Neighbors and internal conflicts within Border.

The Neighbors are not portrayed as one-dimensional antagonists; their motivations and societal structures are gradually revealed, adding depth to the conflict. The series emphasizes tactical combat, with battles relying heavily on strategy, teamwork, and the creative use of Triggers rather than brute strength. This focus on cerebral action sets "World Trigger" apart from many other action-oriented anime. The characters’ development is tied to their ability to adapt and learn from their experiences, with Osamu’s growth as a leader and strategist serving as a central theme.

The plot expands to include larger-scale operations, such as Rank Wars, where Border squads compete to improve their standings, and expeditions into the Neighbor world, which reveal more about the invaders’ society and the broader implications of the conflict. The series balances episodic missions with overarching storylines, maintaining a steady pace while gradually unraveling the mysteries surrounding the Neighbors and the true purpose of Border. Through its intricate world-building, strategic battles, and character-driven narrative, "World Trigger" offers a compelling exploration of conflict, cooperation, and the pursuit of strength in a world under constant threat.