"Argevollen" is set in a fictional world where two nations, the Kingdom of Arandas Alliance and the Countries Unification of Ingelmia, are locked in a prolonged war. The conflict revolves around control of a strategic region known as the "Great Wall," a mountainous area rich in resources. The story primarily follows the 8th Independent Unit, a military group within the Arandas Alliance, as they navigate the challenges of war and the introduction of advanced technology.

The protagonist, Tokimune Susumu, is a young and inexperienced soldier in the 8th Independent Unit. His life takes a dramatic turn when he encounters the Argevollen, a cutting-edge humanoid mecha developed by the Arandas Alliance. The Argevollen is a prototype with advanced capabilities, including a unique AI system that allows it to adapt and learn during combat. Tokimune becomes the pilot of this machine after a series of events, despite his lack of experience and the skepticism of his peers.

The narrative explores the dynamics within the 8th Independent Unit, particularly the relationships between Tokimune and his comrades. Jamie Hazaford, the unit's commander, is a pragmatic and determined leader who often clashes with Tokimune's impulsive nature. Other key characters include Rein Tokimune, Tokimune's older sister and a skilled pilot, and Samonji, a mysterious and highly skilled soldier with his own agenda. The interactions between these characters drive much of the story's emotional and strategic tension.

As the war progresses, the Argevollen becomes a focal point of the conflict. Its advanced technology attracts the attention of both allies and enemies, leading to intense battles and strategic maneuvers. The series delves into the ethical implications of using such powerful weapons in war, as well as the personal struggles of those who pilot them. Tokimune's growth as a soldier and his evolving relationship with the Argevollen's AI system are central to the plot, highlighting themes of trust, responsibility, and the human cost of warfare.

The series balances its focus on mecha combat with character-driven storytelling, providing a mix of action and introspection. The war serves as a backdrop for exploring the motivations and moral dilemmas faced by the characters, as well as the broader impact of the conflict on the world. Through its narrative, "Argevollen" examines the complexities of war, the role of technology in modern combat, and the personal journeys of those caught in the midst of it.