"World Trigger" is set in a world where Earth is under constant threat from interdimensional beings known as Neighbors. To combat this threat, an organization called Border is established, equipped with advanced technology called Triggers that allow users to harness powerful weapons and abilities. The story primarily unfolds in Mikado City, a region frequently targeted by Neighbor invasions.

The protagonist, Osamu Mikumo, is a low-ranking Border agent with limited combat skills but a strong sense of strategy and determination. Despite his shortcomings, Osamu is driven by a desire to protect others and improve his abilities. His life changes when he encounters Yuma Kuga, a mysterious boy who is revealed to be a humanoid Neighbor with extraordinary combat prowess. Yuma joins Border under Osamu’s guidance, and the two form a partnership, with Yuma’s skills complementing Osamu’s tactical mind.

As the series progresses, the focus shifts to Border’s internal structure and its divisions, including the elite A-rank squads and the lower-ranked B-rank teams. Osamu and Yuma, along with their teammate Chika Amatori, form a B-rank squad. Chika possesses immense Trion energy, a resource vital for using Triggers, but lacks combat experience. The trio works to climb the ranks, aiming to participate in large-scale missions and expeditions to the Neighbor world.

The narrative explores themes of teamwork, strategy, and growth as the characters face various challenges, including Neighbor invasions, internal politics within Border, and the complexities of inter-dimensional warfare. The series emphasizes tactical combat, with battles often relying on careful planning and coordination rather than sheer power. The Neighbors are not portrayed as one-dimensional villains; their motivations and the mysteries surrounding their world are gradually revealed, adding depth to the conflict.

Key characters include Jin Yuichi, a high-ranking Border agent with the ability to foresee the future, and Replica, a sentient Neighbor weapon that assists Yuma. The interactions between Border agents, their rivalries, and their shared goal of protecting Earth drive much of the story. The series balances action with character development, showcasing the struggles and growth of its diverse cast as they navigate the dangers of their world and the moral dilemmas posed by their mission.