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The story of the original manga begins after 70% of the moon was suddenly vaporized in an explosion. The super lifeform responsible for this catastrophe is a strange tentacled creature that can slink around at supersonic Mach 20 speeds. It has a 10 billion yen (US$100 million) price on its head, but for some reason, this creature ends up teaching a middle school class at the beginning of the school year. The students have until graduation to assassinate the teacher, or it will destroy Earth as well. imdb.com