The day before the ceremony marking the completion of the Yajima Stadium, Nils Nielsen invites Sei Iori, Meijin Kawaguchi, and others to an exhibition battle using the new large-scale battle system he has designed. Now that they are all reunited, Sei attempts to show off the Build Strike Cosmos that he has upgraded for the exhibition, but then he notices something missing... Meanwhile, somebody takes control of the stadium's systems, sealing Sei and the others inside. Before them appears a mysterious group, which has come to challenge them to Gunpla Battle.
In London, England, unauthorized Plavsky Particles are detected. Using a portable Gunpla Battle system, Takeshi Iori uses his PF-78-1 Perfect Gundam to sneak into a building and open a door, allowing Takeshi and other members of the International Gunpla Battle Referees to bust in, but the room is empty; the people they were looking for weren't there. Takeshi calls Master Chinan and learns of similar occurrences with him and Mr. Ral, and realized it is a diversion.
At the Memorial Park, the former venue for the World Tournament, Meijin Kawaguchi III meets with Ricardo Fellini. Ricardo mentions that due to his experiences at the 7th Gunpla Battle World Championships, he couldn't resist stopping at this place while he was in Japan. Nils Nielsen joins them, reassuring them that the legacy will carry on. At that, Nils invites them to the new 'holy' site of Gunpla Battle, Yajima Stadium. Inside, the Meijin compliments Nils on the new large scale battle system, and they meet with Mao Yasaka, China Kousaka and Sei Iori, whom Nils also invited. Mao compliments Ricardo on his looks, but he teases the kids on their lack of growth, which Mao laments as he's still shorter than Misaki. China thanks the Meijin for coming along, calling him "President Yuuki", which Sei tries to discourage, but Meijin lets it slide.
Mao then reveals that he has a brand-new machine for the upcoming exhibition battle, which makes Ricardo boast of his new Fenice and mentions the Meijin has one as well. He doesn't deny it and suggest that the battle will be a new page in the history of Gunpla Battle, which had almost died out, and noted it should be enjoyed by all in earnest. Nils claims that while he's retired from fighting, he'll participate as well to experience everyone's performance in the new system, though Mao voices his suspicion that Nils simply wants to fight with them. When the Meijin asks what Sei will pilot, Sei mentions the GAT-X105B/CM Build Strike Gundam Cosmos, which has revamped parts for the RG System. However, he finds it missing from its pouch. As he wonders where it disappeared to, China recalls him fawning over a diorama when someone bumped into him. She thinks that person must've stolen the Gunpla.
At that, the lights go out, exits are shuttered and no phone signal can be picked up inside the building. When Sei asks who could have done such a thing, a voice calls out claiming responsibility. On a platform stands five figures - members of the Gunpla Mafia. While Sei and Mao have no clue who they are, Ricardo tells them their background. The leader then reveals that the event is being streamed in real time, and declares that the Mafia will control all Gunpla Battle, starting with the defeat of the world famous Gunpla fighters before them and prevent the relaunch of the Tournament. Viewers of the declaration included Kirara, Misaki and Takeshi. When challenged, the fighters refuse to back down, surprising China. Sei then reveals a new Gunpla he's been building, though not complete - the SB-011 Star Burning Gundam. With that, the battle starts, and a mysterious new battle field made by the leader himself appears.
Setting their Gunpla, the five fighters launch into the new field. Without any idea what they're up against, they can't plan nor can they expect the enemies to play fair. Ricardo's fine with that and blasts open the door to the tower, the others follow him in and they find themselves sent to the Moon. Thinking there's no one around, they boost ahead, but Nils opted to stay behind -there's someone here and his 忍ノ参 Ninpulse Gundam's attack reveals a hidden GM. The Mafia member admits he's impressed that Nils spotted the Gunpla under Optical Camouflage, but boasts a retired fighter is no match for him. Nils easily cuts down the GM, only to learn there are even more GM variants waiting in the wings and he's controlling them.
Further on, the remaining fighters find themselves in a mysterious rose garden before spotting a golden NRX-055 Baund Doc. Ricardo realizes who the Mafia member is and tells the rest to go on as his XXXG-01Wfl Gundam Fenice Liberta transforms to confront the enemy. Ricardo calls out Gawain Oakley by name, and accuses him of hitting rock bottom for joining the Mafia after being fired from Team Nemesis. An angered Gawain retorts that he needed the money to make the most beautiful Gunpla. To Ricardo and Kirara's horror, Gawain's Baund Doc's armor is impenetrable thanks to its diamond coating. However, Ricardo finds a weakness - the Baund Doc's MA mode's bottom is vulnerable, and proceeds to the cut the Gunpla's MS mode head off with the Fenice Liberta's beam tire and strikes it down with one shot. Kirara cheers loudly in her dressing room over the victory, but it is cut short as a mysterious Gunpla takes down the Fenice Liberta which is being immobilized by unknown means.RGMGM-79 GM/GM. Fortunately, China and her KUMA-03 Beargguy III arrives in time and stabs the enemy in its head, and then she tosses the defeated enemy away. Nils then teams up with China to catch up with the others. Further along, after the Meijin destroys a set of turrets, Sei discovers a fork in the road, leading to him, the Meijin and Mao to take separate paths. Mao arrives in a city where his opponent is an SD Musha Gundam Mk-III. Mao prepares to strike down the enemy with his GX-999910 Gundam X Jumaoh, when the Mafia pilot, Nenene reveals her true self - spamming his view screen with her buxom body. In another area, the Meijin confronts "J" and his GM Z'Gok in a cavern similar to the one underneath Jaburo. "J" attempts to appeal to the idea of playing fair, and suggests they used their amphibious Gunpla in a one on one fight in the lake nearby. However, the Meijin quickly calls him out, revealing the "water" is actually paint thinner that melts plastic. "J" attempts to escape, but is quickly struck down by Meijin's MSM-07-A Amazing Z'Gok. Mao's fight with Nenene is going sour as he's distracted by her comical actions. However, after fearing that the Misaki would shun him for liking a woman like that, Mao declares his love for her while blowing away the Musha with a powerful beam that also spelled out his dedication to her.
At the top, Sei reaches the final area as Nils and China find Ricardo. As Ricardo warns them that Sei's Build Strike Cosmos is being used by the Mafia, Sei also figures out the truth. The Mafia leader has set the Gunpla to an auto pilot level beyond the maximum, forcing Sei to fight his own creation. However, Sei realizes that the Build Strike Cosmos has a weakness and exploits it, capturing the suit. The leader calls out Sei for his softness and shuts down the Build Strike Cosmos before revealing his real Gunpla, the massive Psycho GM. Sei fights the enemy, but is easily overpowered, only for the Meijin and Mao to come to his rescue. The leader then cheats, manipulating the Plavsky Particles to immobilize them when Nils, China and Ricardo come to their rescue. Despite this, the Psycho GM transforms and fire its scattering mega particle cannons that pins everyone down.
The leader boasts about the Gunpla Mafia's power, and Sei notes that nobody will acknowledge them even if they win. The leader attempts to finish off Sei when he's suddenly aided by the returning Reiji! Using the Arista on his bracelet, Plavsky Particles fill the the Star Burning as Reiji takes control. Sei tells him to use SP3, which activates the RG System, allowing the Star Burning Gundam to execute the Hyper Star Burning Knuckle, which destroys the Psycho GM. With the battle over, the Gunpla Mafia members lament as to what to do before realizing their boss ran off. Angry that he and his brother's ambitions to rule over Gunpla in public and in underworld are foiled, he attempts to run off to rebuild the Mafia, before Takeshi confronts and arrests him, revealing him as Mikia Mashita, twin brother of the former chairman of the PPSE.
Takeshi thanks the six for their actions as they take the Mafia members away. Sei asks Reiji how he came back, to which Reiji reveals that he borrowed Aila's Arista. Ricardo decides to get everyone dinner for a job well done, with Mao and Nils trying to mooch off of him, but Reiji declines, stating there's one thing he need to do: the fated battle between him and Sei. As the others watch on in awe, the two fighters show their amazing skills, taking their fight from the field of flowers into space, lost in their joy.
One year later, as everyone is preparing to graduate from Seiho Academy, China wonders why Sei was in the Gunpla Club Room. He reveals he had left behind a memory.
- PF-78-1 Perfect Gundam
- SB-011 Star Burning Gundam
- GAT-X105B/CM Build Strike Gundam Cosmos
- 忍ノ参 Ninpulse Gundam
- MSM-07-A Amazing Z'Gok
- XXXG-01Wfl Gundam Fenice Liberta
- GX-999910 Gundam X Jumaoh
- KUMA-03 Beargguy III
- RGMGM-79 GM/GM
- RGM-79HC GM Guard Custom
- RX-79(G) Gundam Ground Type (GM Ground Type colors, with GM Head)
- RGM-79 GM
- RMS-179 GM II
- RGM-86R GM III
- TGM-79 GM Trainer
- RGM-79SC GM Sniper Custom
- RGM-79SP GM Sniper II
- RGM-79N GM Custom
- RGC-80 GM Cannon
- RGC-83 GM Cannon II
- RGM-79FD Armored GM
- RGM-79FP GM Striker
- RMS-179 GM II Semi-Striker
- RX-79BD-1 Blue Destiny Unit 1
- RGM-79L GM Light Armor
- RGM-79ARA GM Juggler
- RGM-89R Jegan A-Type
- RAG-79 Aqua GM
- GM Z'Gok
- Musha Gundam Mk-III
- NRX-055 Baund Doc (Gawain Oakley Custom)
- MRXGM-009 Psycho GM
- LM314V21 Victory 2 Gundam
- XXXG-01W Wing Gundam
- GNX-603T GN-X
- TMF/A-803 LaGOWE
- PMX-003 The O
- Carry-on by BACK-ON (Insert and Ending)
- GM's Counterattack alludes to the movie Mobile Suit Gundam: Char's Counterattack.
- The title can also be interpreted as both the Gunpla Mafia's counterattack and their usage of GM-based Gunpla.
- The dioramas Sei gawks at includes Char's Z'Gok smashing a GM, Last Shooting and the God Gundam holding the damaged Shining Gundam. These same dioramas were seen in episode 6 of Gundam Build Fighters, A Reason to Battle.
- Takeshi's car boasts the license plate "-78-01" which is the model number of the RX-78-01 Prototype Gundam.
- The exterior of the field "Tower" resembles a dropped O'Neill cylinder space colony commonly featured in the Universal Century series.
- Gundam X Jumaoh's beam shooting out of the colony makes it resemble various cannons such as Gryps II.
- The fight between Reiji and Sei starts off in the field where Sei and the Meijin fought during the episode Promise.
- The finale of the fight resembles the final fight between Sekai and Kijima in the episode Final Burst.