Takahashi Mayumi navigates her first year at Sparta Academy burdened by academic struggles. Her innocent nature contrasts sharply with the school's harsh discipline, frequently making her the target of humiliating punishments within the academy's torture chamber. Principal Toenail of Satan and Teacher Ben regularly oversee these sessions, which escalate into sexual humiliation due to her failing grades. Mayumi's peril repeatedly serves as bait to draw out the school's enigmatic protector, Kekko Kamen, who intervenes before violations occur. These rescues foster Mayumi's profound admiration and affection for the heroine, fueling constant speculation about her true identity.

Her observations extend to fellow students. She briefly suspects shy second-year Chigusa Yuka is Kekko Kamen after seeing her unclothed during an exam, a theory complicated by the revelation that Chigusa is actually twins sharing one identity. Mayumi also befriends classmate Koichi Omote, initially perceived as male but later revealed to be female in disguise. After Koichi and her sister Keiko Natsuwata are captured and tortured, Kekko Kamen secures their rescue, cementing Koichi as Mayumi's ally.

The culmination arrives during the final clash with the principal and his forces. The revelation that Kekko Kamen comprises six sisters operating collectively resolves Mayumi's long-standing curiosity. Their coordinated effort defeats the principal and leads to Sparta Academy's shutdown.