Emma Rodman, a commoner from the planet Hydra within the vast Algrand Empire, has dreamed of becoming a knight ever since she witnessed a mobile knight, a towering humanoid weapon, in action. That dream is nearly crushed when she performs poorly in her first battle and is given a humiliating D-rank rating, branding her a failure and leading to her reassignment to a frontier security force on the backwater planet of Alias. Her new post is aboard the light carrier Melea, a ship infamous for being a dumping ground for the empire's most unmotivated and eccentric layabouts.

Initially, Emma’s squad of misfits openly disrespects her, but she is determined to prove her worth as a knight of justice. Her fortunes begin to change when a pirate base is discovered on Alias. During the ensuing battle, her superior officer, Colonel Claudia Beltran, discovers that Emma’s poor performance in standard mobile knights is due to her natural reflexes being too fast for the auto-assist systems to handle. Recognizing her potential, Lord Liam Sera Banfield, the powerful and infamous ruler of the domain, sends Emma a flawed prototype known as the Atalanta, a mobile knight that no other pilot can control but which responds perfectly to her abilities. With this new machine, Emma achieves a major victory, forcing her crew to acknowledge her skills and earning her a promotion to B-rank.

As the story progresses, Emma takes command of the Melea’s squad of slackers and continues her rise from failure to glory. She faces new challenges that test her ideals and leadership, including clashing with an elite knight and former classmate named Russell, who looks down on the Melea and its crew. In another notable arc, Emma travels to the Seventh Weapons Factory to upgrade the Atalanta, only to find that the prototype’s own development team has lost faith in it. A strange engineer known as the Mad Genius even warns against piloting the machine, threatening to halt development entirely. Meanwhile, a mysterious organization begins pursuing the prototype, adding to the dangers Emma must overcome. Later, after training under the top knight Marie, Emma finally unites her ragtag crew just in time to join a full-scale imperial war. However, before they can see battle, the notoriously lax crew of the Melea must survive a grueling inspection by a rigid official who clashes with their unorthodox methods.