Kōga is worried over the disappearance of Athena and frustrated for his failure to protect her and his mentor Shaina. Kōga is determined to master his Cosmo to rescue his goddess. Kōga departs to improve his mastery over Cosmo, later meeting Sōma, the Lionet Bronze Saint, who teaches him about the elements of Cosmo. Eventually, Kōga and Sōma are greeted by the challenge of Mantis Ordykia, one of Mars' henchmen, who puts their command over Cosmo to the test, an experience that strengthens their newfound friendship.

The Saint Fights tournament begins to much anticipation. The young Bronze Saints engage each other in fierce battles, putting their abilities to the test. Kōga manages to attain his first victory after a tense battle, thanks to Sōma's advice; all while his fellow Saints remain intrigued about the seemingly enormous power of Orion Eden.

The young Bronze Saints bravely invade Mars' Babel Tower, and succeed in rescuing the girl he claims to be Athena. She reveals to Kōga that her name is Aria. Sensing a threat approaching, Kōga dispatches Yuna along with Aria to avoid being recaptured, and together, the young Saints try desperately to buy time for their escape by facing the seemingly invincible Leo Mycene. In their darkest hour, a memory of Seiya deep in Kōga's heart brings them hope.

Yuna arrives with Aria to a small town. In Babel, Mars send Sonia (the Highest Martian), The Crow and Miguel of the Hound (Silver Saints) after them. Miguel guides the bad gang to the town where Aria and Yuna are in a church. Aria is left hidden and Yuna goes to fight. Right when Sonia is about to kill Yuna, one martian arrives asking Sonia to go back to Babel for a meeting of Mars with all the Saints. It is revealed that she killed Soma's father. She leaves and the Crow is beating Soma and Yuna. Koga save them and help Yuna to deliver a fatal blow on Crow. In Babel: Yuna, Soma, Koga, Ryuho and Haruto are blamed for Santuary destruction and taking Athena apart. So all the Saints are looking for them.

The Bronze Saints reach the Fire Ruins where Sonia finally comes across the murderer of his father, Hornet Sonia. Souma engages in a giant battle with her but his anger continues to lose his own focus and is unable to hit her. The battle proves fruitless until Koga manages to convince him to control his emotions. This allows Souma to finally hurt Sonia for real and she escapes beaten. After stopping the ruins, Souma realizes that he cannot stay with the team until he has settled things with Sonia and departs on his own.

Haruto, accompanied by Ryuhou, journeys to his home village in an attempt to come to terms with his past and regain his resolve. While there, Ryuhou learns that Haruto became a saint in order to follow the footsteps of his good friend and older brother figure Yoshitomi, the previous Wolf Saint. Yoshitomi had apparently been murdered by a masked silver saint in front of Haruto and the boy had been desperate for answers ever since. Haruto's fater, despite his disapproval of Haruto becoming a saint, helps him gain back his fighting spirit by staging an assassination attempt on his son's life.

The young Bronze Saints decide to invade Mars' stronghold to rescue Aria. The teenagers carry out their plan, finding strong opposition in the enemy lines. During a moment of desperate need, the Bronze Saints receive the assistance of Lionet Sōma, who rejoins them, stronger than before, thanks to further training under Unicorn Jabu, the Bronze Saint. The Saints reach Aria's chambers, only to find she has left. Not all is lost, as Sōma is able to retrieve the Southern Cross Cloth's Clothstone, which belonged to his father Kazuma, after defeating Hornet Sonia, his killer.

After briefly mourning the death of Aria, the Bronze Saints prepare to enter the Twelve Zodiac Temples. They are greeted by Aries Kiki, who explains the situation and how they are to proceed. Taking him for an enemy, Kouga attacks Kiki, to what he responds by restoring the Pegasus Cloth and also revealing to them the crucial secret of the Seventh Sense. Realizing Kiki is not an enemy, Kouga proceeds to the Temple of the Golden Bull, where he is challenged by its guardian, Taurus Harbinger, and a deadly battle ensues. Meanwhile, Kiki restores the remaining Cloths, and stays behind the teenagers, to battle alone against a large group of Martians.

After the defeat of Kouga at the hands of Harbinger, the remaining Bronze Saints engage him in battle, meeting with utter defeat. Harbinger reminisces about his difficult childhood and what drove him to a life of violence, to become the strongest and eventually a Gold Saint. Managing to recover after witnessing the defeat of his comrades, Kouga attacks Harbinger with a single blow imbued with all the power of his Cosmo. Much to his surprise, Harbinger emerges unscathed and his comrades have vanished.

After leaving Eden, Mycenae returns to guard his House, where Haruto and Souma arrive promptly. The young Saints find themselves to fight their way through the House of Leo, but the ferocious Mycenae proves to be an almost impossible foe to overcome, even when the two teenagers fight him together and neutralize his King's Roar technique. When he unleash his King's Emblem, Haruto risks his life by stopping Mycenae's final attack. Souma retaliates with a Lionet Bomber powerful enough to crack one of the shoulder pads on his Gold Cloth. The fierce determination of the ...

Libra Genbu interferes in the battle between Tokisada and the Bronze Saints, teleporting everyone to his temple. To the shock of the Bronze Saints, Genbu takes over the fight and declares his loyalty belongs to Athena, to Saori Kido. While Tokisada is a powerful warrior, Genbu wins the fight. In a desperate gamble, Tokisada attempts a powerful technique to take the Bronze Saints with him but only manages to suck in Ryuho and Haruto. Koga, Souma, and Yuna proceed to the next temple with Genbu's blessing.

The Bronze Saints finally arrive to the House of Pisces, guarded by the ruthless Amor. After separating Kouga and Yuna from the others, Amor summons Mars' strongest warriors, the Four Heavenly Kings to fight the others. While stating that the Heavenly Kings are stronger than the Gold Saints, Amor immobilize Yuna, forcing her to watch her friends being attacked without resisting. Kouga, the only one capable of attacking, is restrained by Amor's Arrested Judgement, a special attack that can only be broken by Darkness Cosmo as part of Medea's plan to have him unleash the...

After infusing the Darkness Cosmo in Kouga's body, Amor finds out that he was betrayed by his sister from the beginning and swears to kill everyone including her, knocking out Eden in the process. Meanwhile on the Earth, Kiki, Harbinger, Fudo, and Genbu use their Cosmos to send Kouga's friends to Mars. As he finally reunites with Saori, Kouga is overcome by the Darkness Cosmo and possessed by the God of Darkness, Abzu, who kills Amor in one single blow. As the possessed Kouga now proceeds to kill Athena with his own hands, his friends try to have him return to his ...