Nothing worries Satomi Oka more than the upcoming final choir competition of his middle school career—right up until he is accosted by a stranger from the shadows who demands, “Let's go karaoke!” As a yakuza, Kyouji Narita doesn't scare easily, but a terrifying prospect has driven him to seek Satomi's help. The boss is holding a karaoke contest, with the loser forced to get a tattoo to be selected and tortuously applied by the boss himself! Oka has been to many vocal rehearsals over the years, but never one-on-one karaoke sessions with a gangster!!

午後雷雨下個沒完，合唱比賽失利的國中生聰實正走出場館，卻被流氓狂兒攔截搭訕，問他想要唱歌好聽該怎麼辦？原來黑道K歌大會將在老大生日當天舉辦，最後一名要成為老大練刺青的人肉畫板，為了避免被評選為唱得最爛，狂兒要聰實幫忙指導歌聲如何改善。捱不過狂兒的苦苦糾纏，聰實越陪練越覺得不能放著不管，還有合唱團和青春期也令他操煩，究竟兩人能否靠著唱卡拉OK解決各自難關？

由于某种原因想要变得擅长唱歌的黑社会成员成田狂儿（绫野刚 饰）邀请担任合唱部部长的中学生冈聪实去卡拉OK指导他唱歌。聪实虽然讨厌狂儿，但还是进行了歌唱指导，在这过程中两人产生了奇妙的友情。合唱部部长冈聪实由试镜选拔而出的斋藤润饰演。

중학교 합창부 부장 오카 사토미는 우연히 마주친 야쿠자 나리타 쿄지로부터 느닷없는 요청을 받는다. 조직 내 노래 대회 꼴찌에게 부여되는 끔찍한 벌칙을 피해야 한다면서, 자신의 노래 선생이 되어달라는 것이다.

合唱部部長の岡聡実（おかさとみ）はヤクザの成田狂児（なりたきょうじ）に突然カラオケに誘われ、歌のレッスンを頼まれる。組のカラオケ大会で最下位になった者に待ち受ける“恐怖”を回避するため、何が何でも上達しなければならないというのだ。狂児の勝負曲はX JAPANの「紅」。聡実は、狂児に嫌々ながらも歌唱指導を行うのだが、いつしかふたりの関係には変化が・・・。聡実の運命や如何に？そして狂児は最下位を免れることができるのか？