Sanshima Koudai is the usual tough high school delinquent, who's always spoiling for a fight. The only person who can hit him and get away with it is his childhood friend Hikaru, who does her best to keep him in line. One day, at the Sanshima produce store, a strange package comes in. Assuming its some sort of rare fruit, Koudai and Hikaru promptly eat it. However, it was much rarer than they could have imagined. Soon after, a childish-looking Chinese "Sennin" (supernatural hermit) who has come to seek it explains that it is a "Taisai," one of only three in the world. It is said that anyone who eats all three gains eternal youth. Eating only one still grants the owner a lifespan of thousands of years, marked out by a winding red line on the forearm. Unfortunately, there are many powerful people in the world who seek eternal youth, and they will stop at nothing to kill those who possess the Taisai and take it from them. Koudai is unshakably confident that he can beat anyone who comes at him, but will he and the nameless Sennin be able to protect Hikaru?

For some reason, there are many incidents around Kirigamine Ryo, the deputy director of the detective department at Koigakubo Gakuen High School. The thief who disappeared from the school building, the relationship between the classmate and the poisoned coffee clan, the UFO turmoil of out-of-school learning-Ryo is enthusiastic about solving it, but the gag is just clear and the reasoning is developing. Is it the deputy director of the detective department who will show off the good reasoning? What is the end of the humor school reasoning?