The world has been divided into two halves by a giant wall to prevent the epidemic of "Dark mist". In order to shelter the refugees, artificial cities called "Arks" were built. Shido Enishi is a private detective, who works at the ninth Ark. One day, he is asked to capture a burglar. The seemingly simple case leads him to a great conspiracy.

An animated short for the Arknights smartphone game's 1-year anniversary.

To save the Earth from alien invaders and their giant monsters, Dr. Kishida does what any other red-blooded mad scientist would do: he builds a giant robot—the ultimate feminine fighting robot! Unfortunately, his grand-daughters refuse to pilot it! Apparently, they've got more important things to do than becoming teenage super-heroes. Besides, how are you supposed to study for your college entrance exams while getting beaten-up in battle? Fortunately for Dr. Kishida, things aren't too well for the invaders, either. According to their reports, the Earth was the perfect target: lush, peaceful, and relatively defenseless. Theoretically, the planet should've surrendered long ago. Now, the war is at a standstill, the dreaded Accounting Department is warning about the serious cost overruns, and the head of the home office has arrived from Galactic Headquarters to personally oversee operations. Has Headquarters decided to unleash, The Audit—or do they have something even more evil in mind? The fate of the world rests upon two people: Aya Kishida, one of the doctor's granddaughters, who may have to give up prep school to pilot the mighty (and lovely) ARIEL; and a mysterious alien named Saber Starblast, who may have the power to defeat the invaders once and for all...

Those incompetent aliens are still plotting to conquer the Earth! The only obstacle in their path is the giant, feminine robot ARIEL! But the young pilots have lives of their own... When Kazumi receives a mysterious love letter from a secret admirer, should she cancel her first date just to save the Earth?