The Prison City, housing over five hundred thousand people, is inescapable. Constantly surveilled by thousands of robot guards, the inhabitants have little chance of jailbreak. Chloe, a seven-year-old girl, has lived in this dreary city her whole life. Abandoned by her parents, she passes her days with her baby brother, Rock. Since the streets are sprawling with dangerous criminals, she has never left the house and finds her purpose in life by taking care of her brother.

One night, after realizing that her neighbors are attempting to escape, Chloe secretly pursues them while carrying Rock. However, when things go south during their escape, she becomes separated from her brother. Faced with harsh reality, she reluctantly escapes without him, holding onto the hope that he will survive until the day she can return. Eleven years later, upon polishing her combat abilities, Chloe returns to the Prison City with fierce determination to search for the brother she promised to protect long ago.