トッペイたちは原人を連れ去ったミカを追って、離れ小島に上陸した。その島には何と死んだはずの熱海教授がいて、凶悪な実験を行っていた。彼は人間を原人化する『マッドパー』という薬品を使って、各地で人間蒸発事件を頻発させていたのだ。そのさらわれてきた人間たちの中にはトッペイの母親の姿もあった。一方、ロックは大西から遺言状を見せられていた。それには「もしミカが無事に戻ったら、ミカと結婚し自分の後を継いで欲しい」と記されていた。ロックはそれならば、と大西の殺害を計画し始める。莫大な遺産を手にするために。
Toppei and the party land on a small island in pursuit of Mika, who has been kidnapped by a primitive savage. To their shocked surprise, Professor Atami, who is supposed to be dead, has been playing malicious games on the island. He has frequently been the cause of mysterious disappearance incidents in various places with a drug called "Mad PA" that can make men regress into savages. Among the people kidnapped is Toppei's mother. In the meantime, Onishi shows his will to Rock. It is written that if Mika is to return safely, he wants Rock to marry Mika and become his successor. Learning of his intentions, Rock starts to plan the murder of Onishi with the aim of getting hold of his immense legacy for himself.
