Hae-Joo Chang has a long history with the "Union Revolutionary Group" and at some point attained the rank of "Commander".Late at night after the death of Yoona-939, Sonmi-451 is wakened from her "sleep box" inside of Papa Song's dinery. She tiptoes to the open door of a storage room to find Commander Chang standing over the lifeless body of Seer Rhee. Sonmi is frightened by the sight and turns to try to sneak away. Chang stops her, telling her there is no need to hide. He introduces himself, telling her he knows who she is. Sonmi asks what has happend to Rhee and Chang tells her that the man had overdosed on "soap," the substance that fabricants drink. There is some implication that Chang may have actually killed Seer Rhee as the dead man's face is bloody and there is blood on the floor around his head.
Chang explains that now that Seer Rhee is dead, Sonmi may be pursued by enforcers and DNA sniffers. He tells her she has a choice. She can take her chances and try to evade the authorities, or she can go with Chang. The two exit the dinery and Chang takes Sonmi up in an elevator high in a skyscraper to show her Neo Seoul. It is Sonmi's first glimpse of the world outside of Papa Song's dinery.
From there, Chang takes Sonmi to a Union doctor (played by Halle Berry). The doctor burns through Sonmi's collar, careful not to set off the small explosive charge that, if detonated, would tear through Sonmi's carotid artery.From they are immediately ambushed by a force resembling a S.WA.T. team, Chang takes most of them down using his extensive hand-to-hand martial arts skills and upon taking a firearm from an officer, uses it to finish off the rest. He and Sonmi-451 continue their trek across Neo Seoul while being pursued by law enforcement, along the way Commander Chang kills the drivers of a truck, and commandeers it whith Sonmi-451 as a passenger, this truck is stopped however, causing him to take Sonmi to the top of a skyscraper. Once on top of the building, Chang extends a device across a gap to another building, he and Sonmi cross the bridge only to be stopped midway by a flying assault vehicle, the occupants of the vehicle begin shooting the bridge out and Chang returns fire, even through his effort, the bridge breaks, Sonmi is captured and Commander Chang falls to his supposed death.
After her arrest, Sonmi requests to know the status of Commander Chang, and is visibly upset when Mephi informs her that Chang is dead. After Mephi's departure an officer turns an kills all the other officers, revealing himself to be Chang, Sonmi embraces him as they continue their trek to the resistance headquarters.After traveling through Neo Seoul's seedy underbelly where Sonmi sees a musician and a replicant like herself being used for prostitution Sonmi and Chang finally reach the Revolutionary Group's headquarters, where she meets with their leader and is brought to Chang's quarters where she sees the now flooded original Seoul. As Chang falls asleep, Sonmi attempts to take in the day's chaotic events, during this time she cuts off the colored strand of hair that marks her as a "replicant". Being unable to sleep, she walks over to Commander Chang and puts her head to his chest in order to hear his heartbeat, this prompts Chang to wake fast and grab her, Chang and Sonmi then engage in intercourse, starting an intimate relationship between the two. in the weeks to follow he and Sonmi enjoy much time together until the Union revolutionary group decides it's time to make their move.
Later in the day, Chang finds two red jumpsuits which he and Sonmi use to disguise themselves as factory workers and enter a very eerie looking facility. when inside Chang takes Sonmi up to a catwalk, when they look down, they see thousands of replicants in white hooded robes marching with candles and singing. Sonmi recognizes this as the release to a better life promised to all replicants,but is informed by Chang that it is not what it seems. Chang proceeds to lead her to a room in which a replicant is seated by factory workers, who promise her that she is ascending to something better, they then execute her, and send her body deeper inside the facility. After being lead to deeper inside by Chang, Sonmi watches in horror as thousands of replicant's bodies are stripped, dismembered, and ultimately ground up. Chang informs her that after reaching a certain age, replicants are sent to facilities like this, and turned into food for the still living replicants, and this is what the Union Revolutionary Group is fighting against. After all this, she's ready to serve the Revolutionary Group.
Under Chang's guidance she writes what she calls her "revelations" and is ready to broadcast it to the world. Chang takes a squadron of Union Revolutionary soldiers and successfully takes over a broadcast station. He and the soldiers barricade the entrance, set up a line and wait for the military to attempt to retake the station. When the military gains entrance, Chang along with his whole squadron are killed after a long firefight, with Sonmi tearfully watching Chang's demise. Before he died defending her, the two exchanged one last glance.