Aria the Scarlet Ammo

Ms. Tsuzuri takes her students to a hot spring in order to undergo Butei training. She tells them that their training starts the next day; however, strange things start happening on their way back to their rooms after enjoying the hot spring bath.

Kinji Tohyama is a student at Tokyo Butei High school, where special detectives known as Butei are trained to become mercenaries. Whilst riding his bike to school after missing the bus, Kinji is chased by a remoted controlled segway armed by an uzi, telling him that a bomb has been attached to his bike which will explode if he slows down, also threatening to shoot him if he jumps off. Kinji is saved a mysterious twintailed girl named Aria H Kanzaki. As the two end up pinned down in a gym storage locker as more segways appear, Aria fends them off, sticking her chest right in Kinji's face whilst doing so. The high blood pressure causes Kinji to enter his 'Hysteria Mode', where he manages to perfectly destroy the segways, as well as counter Aria's moves when she complains about him being a pervert. As Kinji finally arrives at school, he learns that Aria has become a transfer student in his class. Later that day, Aria comes to Kinji's house and declares him her slave.

Aria demands that Kinji join her Butei party, intruding in his living space when he refuses. This puts Kinji in an awkward situation when his childhood friend, Shirayuki Hotogi, pops over whilst Aria is taking a bath. The next day, Aria follows Kinji around as he takes on a quest to search for a missing cat. After managing to lose Aria, Kinji trades some eroges with his friend, Riko Mine, to obtain information on Aria. Meanwhile, Aria calls Kinji's bluff on his Butei abilities and coerces him to agree to join her on a single Assault case, so she can witness his Hysteria Mode.

Aria follows Kinji to the arcade where he manages to win some plush dolls for her. The next day, Kinji and Aria learn that a schoolbus has been hijacked by the fabled Butei-killer and fitted with a bomb that will explode if it slows down with an armed convertible preventing the passengers from interfering. As the busdriver gets injured when an attempt to stop the convertible is foiled by heat-detection, Aria and Kinji arrive on the scene, taking out the convertible. With the bus slowly running out of gas, Aria finds the bomb on the underside of the bus whilst Kinji finds and destroys the communications sensor. The bomb is disarmed by the sniper ace, Reki, but Aria is injured by another armed vehicle whilst trying to protect Kinji. As she is treated in hospital, she feels disappointed that Kinji didn't meet her expectations.