High school student Izumi Hoshi loses her father suddenly in an accident. One day, Sakuma, the waka gashira of a small and weak Yakuza gang in Asakusa - the Medaka Gumi - pays Izumi a visit along with the gang members. The death of their former kumicho (Boss) puts Medaka Gumi on the verge of a breakup. Therefore, Sakuma begs Izumi, the former boss's only distant relative left, to succeed in the kumicho position. Izumi is taken by surprise and keeps refusing at first. However, she ends up becoming the 8th kumicho of the Medaka Gumi! The police tell her that her father's death may possibly be a homicide case. There's also Mayumi, a mysterious woman staring closely at Izumi's father's photograph at his funeral. The situation becomes complicated when the drug-dealing vicious yakuza, Hamaguchi Gumi, comes into the picture, as well as Sandaiji Hajime, a politician whose public stance is to eliminate drugs. ~~ Adapted from the novel series "Sailor Suit and Machine Gun" (セーラー服と機関銃) by Akagawa Jiro (赤川次郎).