X-7, a Mysterious Enemy Agent

One night a huge balloon comes down in the Ginza and lifts away a big building. Azuma who rushes there receives a challenge letter from a man who calls himself X7. Nothing has been known about X7, except that X7 is a very good spy of BF Syndicate. Next day, a boy called Kuredajoins Daisaku's class in school. He invites Daisaku and two boys into his house where he is living with his uncle after his parents died. Following them, Minami inhales poison gas in a taxi and faints. In Kureda's house all the children including Kureda faint likewise. By Ippei, Kureda's uncle, and Red Cobra, they are taken to the basement where Minami is imprisoned. Ippei seems to be X7 himself. Kureda asks their forgiveness saying that he didn't know his uncle was such a dreadful spy.