Without spoiling much, you're the son of a Yakuza leader, and you're still in high school. You meet a bunch of girls because Japan, and various shit goes on in these girls' lives. You live a really comfortable life with a hella awesome apartment (especially when you consider it's in Japan, where space is at a premium), all due to your dad's connections. Your friendship with these girls is tested by the shit you need to do as a Yakuza's son. You are often put in situations where you risk losing everything if you go against your father's wishes, because he has provided you with everything.
In addition to this, a character named "Maou" (devil) is also in play, and has directly challenged the Yakuza. Your dad tasks you with finding out who Maou really is, and stopping him from fucking with the Yakuza. One of the girls is early on clearly some sort of ace detective, and she has taken an interest in the Maou case as well.