Contains spoilers for this episode and earlier episodes.
Lee Byeong-chan finds his son Jin-su in hospital after a brutal attack by other students. The boy is still alive after falling from a rooftop, but his body twists and he threatens the people around him. Byeong-chan strikes him with a Bible, then discovers that even this does not end what is happening. At Hyosan High, his science laboratory holds another infected creature, which bites the student Hyeon-ju when she investigates its cage.
Byeong-chan locks Hyeon-ju inside the laboratory instead of seeking ordinary medical help. Elsewhere, On-jo spends the school day with her childhood friend Cheong-san while nursing a crush on Su-hyeok. Gwi-nam and the other bullies continue tormenting Eun-ji and Cheol-su, despite Su-hyeok's attempt to intervene. The same school that has failed to protect Jin-su carries on with its lessons while Byeong-chan tries to conceal the danger in his room.
Hyeon-ju escapes and tells the school what her teacher has done. Paramedics take her to hospital, and police arrest Byeong-chan when he tries to insist that she must be isolated. She has already bitten the school nurse, who becomes violent and attacks a student. Infection spreads through the building as students flee their classmates. In the cafeteria, On-jo is caught in the crush of attackers, with Cheong-san and their friends struggling to reach safety.