Na Yonsu and An Sangeen arrive at Ye Ran High School to confirm the identity of M-21. While An Sangeen is discovered by Takeo resulting in an unavoidable conflict, Na Yonsu starts her fight with M-21.
Sangeen and Yonsu decide to infiltrate the school after class hours. An Sangeen puts the few staff left inside to sleep but clearly expresses his worry about the plan while they both put on masks to keep their faces hidden. Yonsu appears before M-21 and their fight immediately starts.
While An Sangeen observes from the roof, Yonsu leaps down and appears before M-21 who is doing his rounds around the school. M-21 inquires after the identity of the intruder but realizes that he shouldn't have asked seeing as keeping the identity hidden is the purpose of the mask that Yonsu is wearing.
Their duel starts with Yonsu striking first by aiming a kick at M-21's head. M-21 crouches low and throws a punch at his opponent while Yonsu analyzes his movements. M-21 strikes again and Yonsu blocks with her arms but the force pushes her back. She decides to use her enhanced strength to attack but M-21 easily dodges, showing Yonsu his reaction speed. Yonsu tries to kick him but M-21 blocks it with his arms and leaps back to reduce the impact. Yonsu tries to attack with greater strength and speed but M-12 remains standing even after receiving the blow. Yonsu increases her strength again and M-21 is pushed back towards a wall. The KSA agents briefly think that Yonsu may have killed him but when the smoke from the impact clears, M-21 is revealed standing up and brushing the dust off his uniform. M-21 then states that he has realized the identity of the intruders. Sangeen and Yonsu confirm that M-21 is not an ordinary human being as nobody should have survived that kind of attack.
Takeo appears behind Sangeen and a battle ensues between the two. Meanwhile, the battle between M-21 and Yonsu continues. Yonsu successfully lands a punch on M-21's face followed by a punch on his stomach. Another blow landed on a building, damaging it and M-21 is momentarily distracted, thinking about the school. He takes another hit and blocks it. He draws out his claws and slashes at Yonsu who backs off just in time. Seeing Yonsu's speedy escape and M-21's physical transformation, the fighters simultaneously ask each other if they come from the union. The question surprises both of them. Sangeen suggests to Yonsu that they withdraw just as Tao arrives, the three now completely surrounding the pair.
Yonsu takes off her mask and reveals herself to the trio. Sangeen dejectedly pulls his own mask off and they all negotiate about their situation. Tao reveals their knowledge about the pair's affiliation with the KSA and announces that their reason for being at the school is for keeping a low profile. Tao also cleverly guesses that Yonsu and Sangeen's modification is a secret even from the KSA. This leads to his suggestion that both parties keep each other's secret and Sangeen accepts the deal for the time being.