Doctors Kazamatsuri and Onizuka are geneticists researching cures on diseases such as AIDS and cancer by performing experiments to strengthen the human body. The test subject is Shin Kazamatsuri, the motorcycle racer son of Doctor Kazamatsuri. Unknown to them, their operation is funded by a syndicate group, who plan to use this research to have the bodies of men strengthened for their own gain—they have already been experimenting in the field of cyborg soldiers, to a less-than-successful attempt. However, they did not count on Onizuka's own secret ambitions; the mad Onizuka wanted to create a new species, by fusing a grasshopper's DNA with the test subject's, in order to start a new civilization and be their god. He has tested on himself, but seems to be having greater success with Shin.
Meanwhile, a murderous creature is stalking the city, while Shin dreams of it. While he believes he is the one causing the murders, he eventually finds out Onizuka's plan and discovers that it's Onizuka who has been behind the killings. Onizuka has experimented on himself and altered his genes, making him a humanoid grasshopper; the telepathy of grasshoppers allowed him to communicate with Shin, resulting in his witnessing the murders.
The syndicate learns of Onizuka's plans and have him institutionalized. As if the syndicate were not enough, a CIA agent tracks Shin and wants him eliminated, for he doesn't know the true threat he could pose. The repulsed Shin digs to find out all he can about the experiment he foolishly volunteered to be a part of, which is now altering his body and putting him and his loved ones in danger.
風祭真は父の大門が働く生化学研究所、ＩＳＳ社で生体実験の被験者として生活を送っていたが、連続殺人事件の全てを記憶しており苦悩していた。ＩＳＳ社の鬼塚が変身し殺人を繰り返し、そのデータを真の体に適用し、真の体は知らぬ間に改造兵士（レベル３）になっていたのである。ＩＳＳ社は所長氷室の指揮のもと、改造兵士の開発を企んでおり、真の肉体と精神は、鬼塚の改造実験には最適だったのだ。一方、セーラ・深町率いるＣＩＡは改造兵士開発を阻止するため鬼塚を襲う。炎の中で身もだえする鬼塚に呼応して、ついに真が変身する。氷室の腹心、豪島もレベル２の兵士として真の前に立ちはだかる。改造兵士同士の激しい戦いが、ＩＳＳ社の中で繰り広げられていく。一方、ＩＳＳ社の明日香愛は真の子を身ごもっており・・・
內容圍繞主角風祭真和財團之間的恩怨，真的父親是臨床免疫學權威博士，因被「財團」的ISS所長欺騙，為其進行研究，而真為了幫助父親研發出不治之症的藥物，毅然自願成為人體實驗者，卻不知道這一切都是「財團」的陰謀，他們的真正目的是研究生化戰士，並已暗地裡將真改造成蝗蟲生化人。後來，每當真情緒激動，便會演變成半人半蝗蟲的怪物模樣…
