Mirio tries to break the ice with a joke but gets a cold response. Mirio figures the first-year students have little ideas about what's going on, so he decides to explain by showing rather than telling. He challenges all of Class 1-A who earned their provisional licenses to fight him all at once.
Everyone changes into their gym clothes and moves to Gym Gamma. Tamaki tells Mirio not to kill the younger students' spirits. Eijiro and Fumikage suggest that they don't need a handicap from Mirio and the latter tells them to attack whenever they're ready. 
Eijiro tries to attack first but Izuku steps up to the challenge and activates One For All: Full Cowl. Members of Class 1-A specializing in close combat decide to surround their opponent. Everyone arms themselves with their Quirks until they're taken by surprise when Mirio's clothes suddenly fall off his body. He rushes to get his pants back on and Izuku kicks at his head. Izuku's foot phases clean through Mirio's head and the former confirms Mirio's Quirk allows him to pass through things.
Mirio turns his attention to Izuku and gets attacked from behind by Yuga, Hanta, and Mina's Quirks. Their attacks slip right through Mirio's body and he disappears in the ensuing explosion. He suddenly reappears behind the class and claims he'll take out the long-range fighters first. Fumikage uses Black Abyss to try and attack Mirio from behind, but he slips through the ground and hits Fumikage in the gut with a powerful surprise attack. He quickly moves in and knocks out Hanta and Minoru with two quick punches. Yuga, Mina, and Tsuyu try to subdue him together but end up knocked down by his quick attacks.
Using a combination of his Quirk and technique, Mirio defeats half of Class 1-A within a few minutes; Mezo, Denki, Kyoka, Mina, Fumikage, Momo, Tsuyu, Yuga, Minoru, and Hanta. Shota tells Class 1-A to take this great opportunity and learn from it because as he knows it, and reveals that Mirio is the closest hero to being No. 1 while even including the Pro Heroes.
Having dealt with the long-range combatants, Mirio turns his attention to the close-ranged combatants of Class 1-A. Eijiro is shocked that Mirio has a Quirk that can make him invincible, but Izuku rallies his remaining classmates and says they need to wait to counterattack once Mirio comes to them. Whatever Mirio does, he will use a direct attack and during that moment they should counter-attack. Eijiro thanks him for the tip since it might work.
Mirio charges and slips through the ground. Mirio rises up behind Izuku, but he predicts this and delivers a One For All Full Cowl-enhanced kick toward Mirio's face. Mirio is impressed with Izuku's perceptive skills and uses his Quirk to pass through Izuku's attack. He fakes Izuku with his Super Move Blinder Touch Eyeball Crush technique and then knocks him out with a swift punch to the gut. Mirio comments that everyone counters him that way so he naturally trained to counter back.
Tenya calls out to Izuku, but before he can do anything, Mirio quickly moves on and appears behind Tenya and hits him in the stomach, defeating him. Then he knocks out Eijiro, Koji, Rikido, Mashirao, Ochaco, and Toru. Mirio stands victorious and yells "power" while all the first-year students lie knocked down in pain.
Nejire asks Tamaki if Mirio's gotten stronger and the former comments that Mirio needs to learn more restraint. Mirio asks Class 1-A if they understand more now, but Izuku says they haven't learned anything by getting punched in the stomach. The students claim Mirio's Quirk is too powerful and ask about its nature. Nejire and Mirio explain his Quirk: Permeation gives him the ability to phase through anything, including the ground.
Mirio explains the drawbacks of his Quirk and how he had to train to make his Quirk strong. Experience gained through a work-study gave him the ability to fight with his head and learn how to properly utilize his Quirk. Mirio wanted to fight Class 1-A to show them this through experience rather than simply telling them about work-studies. The class thanks the Big 3 for explaining and the third year students exit the gym.