As they arrive to Mars, they learn that the Mars Branch is withholding support from McGillis as he has been officially stripped of his rank and title back on Earth. With orders from Orga to hold. The two quickly shoot each other, but Gaelio's mask protects him while McGillis is fatally shot in the chest, with Gaelio pinning the dying McGillis to a wall and demanding acknowledgment as the victor. Atra contemplates having a baby with Mikazuki herself. Upon being asked if what they are doing is on Orga's orders, Takaki hesitates but then recklessly attacks McGillis, only to get outmatched by the Graze Ritter. Meanwhile, the Hotarubi and Gundam Flaruos is severely damaged and Shino is injured by the Dáinsleif squadron's second barrage.
Meanwhile, Gjallarhorn's garrisons in Arbrau have been forced to remain on their bases, unable to intervene or report outside Arbrau. Tekkadan and McGillis retreat from the mining plant, while Iok is forced by his comrades to fall back as well. While McMurdo declines to break the cup due to his investments on Mars, he warns Orga that any betrayal of Teiwaz would lead to severe consequences for Tekkadan. Furthermore, Nobliss Gordon has ceased funding Admoss Company and will only restore it if Kudelia cuts her ties with Tekkadan. Disillusioned by the political intrigue and bloodshed in Rustal's actions, Julieta asks what drives Mikazuki to fight even in a hopeless battle, to which Mikazuki states he has none beyond survival. After revealing himself under the mask, McGillis tells Orga and his team his plans to reform the corrupt Gjallarhorn.
Meanwhile, McGillis arrives in a Montag mobile suit to confront Gaelio. She also opens an orphanage at the Sakura Farm on Mars. The powers which ruled the Earth Sphere have been overthrown, giving way to new rulers of a new world, exhausted of battle. Rustal refuses to agree with Iok due to a lack of evidence to support his argument, allowing McGillis to claim credit for slaying the Hashmal. This is a subversion of the usual state of affairs in a Gundam series, where a hit with a beam weapon on a non-main character machine is usually devastating. To make matters worse, all transportation, diplomatic, and military access into Arbrau have been blocked by control of Galan and Radiche, making negotiations or entry for either Tekkadan or McGillis impossible. Akihiro, Shino, and Ride arrive at the scene to assist in the evacuation while Naze sends the Hammerhead on a kamikaze run toward the fleet.
Aston sacrifices himself to save Takaki and trap McGillis before Mikazuki arrives and intervenes in the melee, forcing Galan and his troops to retreat. Both reminisce on and express their regret over the tragic ends of McGillis and Tekkadan, though resolve to keep moving forward. Refusing his order, Tekkadan battles Teiwaz in Mobile Suit combat, but unaware to them, the Mobile Suit battle is a distraction for several Tekkadan troops led by Orga to infiltrate and hold the Teiwaz members as hostages. The Hotarubi is used as a shield for the Isaribi to allow Shino a chance fire the Super Galaxy Cannon at Rustal's ship - but unfortunately, Julieta manages to throw off his aim, causing his shot to narrowly miss the bridge of Rustal's ship. It is later revealed in Season 2 episode 10 that beam weapons are ancient technology rendered completely ineffective against mobile suits due to their armor.
Makanai agrees to help Tekkadan with their new identities, but they must return to Earth to finalize the process. Meanwhile, Jasley has his men spy on Tekkadan and McGillis' activities, before informing Rustal's fleet of McGillis' secret trip to Mars through his connections with the Kujan Family. The Earth Sphere had lost its previous governing structure, and a new world was created under new systems of government. This enrages Gaelio, who attacks relentlessly until McGillis stabs Gaelio's Kimaris suit through the chest, killing the latter. As Tekkadan struggles to repel the Gjallarhorn forces, Rustal notes McGillis' story will be remembered as a tragic consequence for those thirsting for power and sends Julieta out to finish Tekkadan off.
Just as Lafter is shopping for a gift for Akihiro, she is shot dead by one of Jasley's subordinates. Nevertheless, Tekkadan's precise tactics and Mobile Suit squadron mow down majority of the enemy fleet, but the Dawn Horizon Corps strike back with Mobile Suits of their own with many piloted by disposable Human Debris. With the help of members of a local news network, Orga and his team re-establish communication with the Isaribi while Mikazuki flies out to space to once again pilot the Gundam Barbatos and engage the Gjallarhorn forces in the conflict. Upon receiving word that his elder brother Savarin has committed suicide for his actions that resulted in a Dort 3 massacre, Biscuit contemplates on leaving Tekkadan. At the end of the episode, the second season, which will begin airing in Fall, 2016, was teased.
As they try to figure out how to support themselves financially, Crank returns to the base to issue a challenge to the Gundam Barbatos in order to retrieve Kudelia and the Graze captured during Gjallarhorn's previous assault on the base. Before they can come to blows, Mikazuki intervenes the rebuilt Gundam Barbatos Lupus Rex and battles the Gundam Vidar, but Gaelio reveals that his Gundam uses a pseudo Alaya Vijnana System based on Ein Dalton's brain. Following the incident, Orga and McGillis promise to help each other take down Iok and Jasley to achieve their long-term goals. While they succeed in stopping the invasion force, Carta tries to kill Orga, whom she realizes is leading Tekkadan. As McGilis rallies his troops, Gaelio attacks him with the restored Gundam Kimaris Vidar. To reconcile with their feelings over the death of their comrades, Tekkadan's conduct a funeral in the memory for those who fell in the battle before proceeding as planned.
Such brutal actions sadden Kudelia, as she hoped to build a world free of bloodshed for them but is making minimal progress. Meanwhile, Kudelia offers to remedy the illiteracy of Mikazuki and the younger crew members by teaching them to read and write. As Tekkadan dispatch the rest of the Plumas, Mikazuki stops the Hashmal from killing McGillis, Isurugi, Ride, and Julieta, risking his body to unleash the Gundam Barbatos Lupus' full potential by releasing its limiters. Later, Tekkadan celebrate their success in one of the vessel's commercial properties to relieve their stresses. As Tekkadan prepare to corner the Hashmal in a gorge, it changes its course toward the public agricultural plant after being fired upon by Iok. Later, Eugene and Akihiro corner Radiche, demanding Galan's whereabouts. Mari Okada and other staff members protested the idea and had the ending rewritten as a result.
While largely forgotten, Tekkadan's surviving members live on with new identities in the Admoss Company, Sakura's orphanage, the Martian Union and the Kassapa Mobile Worker Factory, with all of them remaining in touch. With time running out, Orga sorties Shino and Yamagi in the Gundam Flauros to separate the Mobile Armor from its Plumas with the Galaxy Cannon, but they fail to slow the Hashmal itself. Vidar suspects that McGillis is out to destroy the Mobile Armor to earn the Order of the Seven Stars; the highest honor within Gjallarhorn. The company accepts a mission to escort , a girl who seeks to liberate the Martian city of from the rule of one of Earth's major powers. The first season aired every Sunday at 19:30 in Japan from October 4, 2015 to March 27, 2016. The demonstration becomes deadly when Gjallarhorn forces intervene and Kudelia finds herself involved with the Union. As the series comes to a close, Kudelia silently monologues that while Mikazuki and Tekkadan are gone, she could still love the world that they sacrificed their lives to change.
Humiliated by McGillis, feeling betrayed by Rustal's refusal to defend him and obsessed with his honor and his men's sacrifice, Iok orders his servants to contact Jasley. As McGillis broadcasts his speech worldwide, he is interrupted by Gaelio, who declares his intent to defeat his former comrade. One month later, Orga reports to McMurdo about the incident and Gjallarhorn's searching for other Calamity War remnants on Mars. In ensuing battle, the two ships successfully incapacitate the enemy while the latter's mobile suits are gradually eliminated. Almiria confronts McGillis over the truth about her brother Gaelio and threatens to kill him, but he instead stops her from committing suicide. Meanwhile, McGillis and Gaelio returns to Earth and assess the situation within a Gjallarhorn. Earth's influence on Mars erodes after Rustal reduces the size of Gjallarhorn's Mars branch, while the cities of Mars gain independence and form the Martian Union with Kudelia as their first chairperson.