Asta vs. Mars is a fight that occurs during an exploration of a newly emerged dungeon.
After Asta manages to neutralizes Mars' attack that was about to kill Yuno, the two foster brothers begin to argue whether the latter is in serious situation or not before agreeing to fight together. On the other end, Klaus is surprised on Asta's sudden appearance that he fails to see Mars' puppet that is about to attack him. Luckily, Luck Voltia arrives just in time to save him in which put him in deep shame.
Subsequently, Noelle Silva also discovers her cousin is lying within a spell and trying to recuperate from Mars' previous attack. Without judging or belittling her, Noelle immediately cast a protective spell around Mimosa Vermillion as she prepares for the upcoming fight.
Seeing that he is outnumbered by the Clover Kingdom's mages, Mars creates an army of puppet based on him in an attempt to fight against all of them at the same time. While five of them are occupied with the puppets, he focuses his attention on Asta and swings his huge crystal sword at him. However, Asta easily breaks any spells that Mars launches at him and sends the latter flying to the nearby pool.
Mars quickly regains his footing by creating a huge walking armor around him and uses it to attack Asta. The attack manages to injure the latter but it is not strong enough to put him down. Seeing his opponent's high vitality makes Mars curious about Asta's identity. The latter replies by saying that he is a mere human without any magical powers with an ambition to become the Magic Emperor. Asta's answer triggers a memory on Mars in which the latter tries to brush it off by attacking Asta once again. Unfortunately, Asta is able to breakthrough Mars' attack and render him unconscious.
Once Mars losses his consciousness, Klaus Lunettes immediately puts a Restraining Magic spell on him before proceeding to enter the dungeon's treasury. While the Clover Kingdom's mages are exploring the treasury, Mars regains his consciousness, activates a fire-based healing spell and storms into the treasury. Once inside, Mars quickly renders Klaus, Yuno and Luck immobilized while also inflicting a critical damage on Noelle whom tries to negate his healing spell.
Mars then shifts his attention towards Asta whom is charging towards him. The Diamond Kingdom's mage reveals that he already has a full understanding on Asta's fighting style as he launches a high-speed attack. The said attack manages to send Asta flying towards a hidden room behind a wall. Inside the room, Asta finds a sword at which Nero has been hinting since they entered the treasury.
Outside of the hidden room, Mimosa sees her cousin suffering a critical injury and begins to heal her immediately. Her action is noticed by Mars who tries to finish both of them. Fortunately, Asta intercepts the attack just in time with his newfound sword while the other mages are still bound inside Mars' spells. Asta begins his assault and is able to overcome Mars' high-speed attacks. However, Mars' healing spell is hindering Asta from inflicting any significant damage. At that moment, Noelle speaks to him as she regains her consciousness. The noblewoman reveals her acknowledgement of Asta and orders him to finish the fight in which the latter complies.
Asta's new sword begins to glow and when he swings it towards Mars, a stream of water comes out which neutralizes Mars' upcoming attacks and douses his fire-based healing spell. Having seen this new attack, the others postulate that the sword was absorbing Noelle's magical power and using it as its own. Unfortunately, Mars' attacks were not completely neutralized by Asta's water-based attack as one of them is piercing through the latter's stomach. Upon Asta's fall and defeat, Mars immediately prepares to kill him by creating another huge sword.
Seeing Mars' attempt to kill Asta, Klaus and Luck immediately hasten their attempt to break out of Mars' binding. Yuno manages to successfully break his binding but he does not have a spell in his arsenal that could save Asta from his predicament. However, as he refuses to let Asta dies, Yuno's surrounding suddenly becomes motionless. He then sees a girl with a pair of wings appearing right by his side. The said girl then blows a gust of wind towards Mars in which resulted in a devastated attack on Mars as Yuno's surrounding returns in motion. When Klaus and Luck's binding dissipate into thin air, he confirms that Mars has been truly defeated.
Moments later, the girl who delivered a devastating blow to Mars disappears as the writings that Yuno found on the scroll are appearing on his grimoire. At the same time, Asta's newfound sword enters his grimoire and unknown writings are also appearing on it as well.
Subsequently, Noelle quickly requests Mimosa to attend to Asta's injury while she is still struggling with her own injury. However, before Mimosa could reach Asta's side, the dungeon begins to collapse in which the locations where the scroll and the sword were located are the initial points of rupture.