The Build Divers examine their new Force Nest, a cabin on the water shaped like a boat. They look at the various rooms, including the mobile suit catapult built into a mountain. Yukki thinks they’ve gotten as far as they have because everyone else was great. He didn’t think when he started out in GBN that Riku would be able to build such an amazing Gunpla. As Yukki walks from school to The Gundam Base, he wonders about his own progress and thinks to himself that he wants to get stronger. He tells Nanami about their new island and suggests that she visit it. In GBN, Yukki tells Magee that he wants advice on how to become as strong as Riku to prepare for the Raid Battle. Magee then suggests the mirror mission, but he doesn’t explain what it’s about, other than to say it is full of trials and hardships. Magee notes that he, Tigerwolf, Shahryar and Kyoya all benefited from completing the mission. Yukki then goes to accept the mission and finds that Do-Ji also wants to sign up for it. Sarah shows up and is pulled into the situation when Molly presses the “accept” button on their behalf. In a cave, Yukki reads the instructions, which state that the mission has four challenge phases, followed by a final phase. Yukki vows that he’ll beat the challenges and get stronger, but Sarah tells him he’s already strong. Do-Ji wants to go off on his own, but Yukki tells him they all have to complete the challenge together. A bird character watches them from above. In the first phase, the players are challenged by NPD monsters, so Yukki and Do-Ji summon the GM III Beam Master and Geara Ghirarga. Yukki finds himself at a disadvantage in close range combat, and Do-Ji is attacked by several monsters while Sarah plays tick-tack-toe with an SD character. They then walk to the area for phase two and find themselves in a batting cage with a baseball-customized 0 Gundam. A torrent of balls fly their way, but Yukki and Do-Ji use mitts to catch them while Sarah washes balls. Phase three presents them with multiple puzzle rings, and Do-Ji wonders how this will help with Gunpla Battle. The fourth phase takes them through an obstacle course, and Sarah slips off a narrow bridge. Yukki and Do-Ji catch her but then lose balance and fall in themselves.
The three sit in a cave while Yukki and Do-Ji’s clothes dry off. Do-Ji comments that Ogre also respects Riku and is working on a new Gunpla. Do-Ji says that Ogre is honing his skills to experience the ultimate battle, and Do-Ji feels like he’s falling behind. But no matter how hard he worked, Ogre kept pulling ahead, so he resorted to using a Break Decal. Yukki understands, even though for him it’s the opposite with Riku starting out weaker and then advancing past him. Sarah says that everyone knows how they feel and love their Gunpla. After everyone properly introduces themselves, Do-Ji awkwardly apologizes for the trouble he caused with the Break Decal. Do-Ji wonders if he could support his force better by switching to long-range style, and Yukki thinks about switching to close-range. For the final phase, Yukki and Do-Ji have to fight black versions of their own Gunpla. Each gives the other tips on how to fight in their new combat style. Sarah faces a mirror version of herself wearing a black dress. Do-Ji feels like he’s fighting himself, and Yukki wonders if the enemies are using their past data. The mirror Sarah asks Sarah who he is and vanishes when they touch hands. The mission then glitches, with the black Gunpla combining into a strange monster. The monster attacks Sarah, but Do-Ji rushes in to slash at the tentacles. Yukki blasts several tentacles and realizes how the previous challenges helped them hone their skills. Yukki then provides cover fire while Do-Ji rushes in to attack the monster. He realizes that a large eye is a decoy and blasts the tail to destroy the monster. The bird character appears before them before flying away. Back in the GBN lobby, Do-Ji apologizes to Riku for tricking him and for later using a Break Decal. He then boasts that no matter how strong Riku gets, Ogre will always be stronger. Do-Ji tells Yukki that he doesn’t have to change everything, but she should change the things he ought to. At The Gundam Base, Yukki starts building a new Gunpla using a Jegan as the base. Elsewhere, the Game Master is informed about a glitch on the mirror mission server. He tells his staff to keep on watch for more bugs.
Yukki and Do-Ji both feel that they are falling behind their teammates and go on a journey to increase their strength that ends with them having to face themselves. Yeah, it’s a completely cliched concept that is executed exactly how you’d expect. The only wrinkle is Sarah touching her doppelganger and creating glitches that lead to Yukki and Do-Ji fighting a weird Gunpla monster. The mysterious bird has been watching them for awhile now, and glitches related to Sarah are now on the Game Master’s radar. Maybe now the characters will finally realize what was obvious from the beginning, which is that Sarah isn’t a “real” person, but a creation of GBN.