"Aria the Scarlet Ammo" is set in a world where elite schools train students in specialized combat and investigative skills. The story centers on Kinji Tooyama, a seemingly ordinary high school student at Butei High, an institution dedicated to training "Butei," or armed detectives. Kinji possesses a hidden ability known as the "Hysteria Mode," which activates under extreme stress, enhancing his physical and mental capabilities to superhuman levels. However, this power comes with a significant drawback: it triggers uncontrollable romantic impulses toward women, complicating his interactions.

The narrative begins when Kinji encounters Aria H. Kanzaki, a skilled and headstrong transfer student from England who is also a top-ranked Butei. Aria is a descendant of the legendary detective Sherlock Holmes and wields dual Colt M1911 pistols with exceptional precision. After a series of events, Kinji becomes Aria's partner, and the two form a contract to work together on dangerous missions. Their partnership is marked by a blend of tension and camaraderie, as Aria's fiery personality often clashes with Kinji's more reserved demeanor.

As the series progresses, Kinji and Aria take on various missions, ranging from solving crimes to thwarting terrorist plots. Along the way, they encounter other Butei students, each with unique abilities and backgrounds. These include Riko Mine, a cheerful and resourceful hacker; Reki, a stoic sniper with a tragic past; and Shirayuki Hotogi, a swordswoman with a deep affection for Kinji. The relationships between these characters add layers of complexity to the story, as alliances and rivalries shape their interactions.

The overarching plot involves a mysterious organization known as "E.U.," which seeks to destabilize the world order. Kinji and Aria's investigations gradually uncover the group's sinister plans, leading to high-stakes confrontations. The series balances action-packed sequences with moments of humor and character development, exploring themes of trust, loyalty, and the burden of responsibility.

Throughout the series, Kinji struggles to reconcile his desire for a normal life with the demands of his Butei duties and the unpredictable nature of his Hysteria Mode. Aria, meanwhile, grapples with her own insecurities and the weight of her family legacy. Their partnership evolves as they face increasingly dangerous challenges, culminating in a climactic battle that tests their resolve and strengthens their bond.

"Aria the Scarlet Ammo" combines elements of action, mystery, and romance, creating a narrative that appeals to fans of multiple genres. The series' focus on character dynamics and its exploration of the Butei world provide a compelling backdrop for its fast-paced plot and engaging storytelling.