When zoologists look into a giant monkey reported to be swinging from cables in the Amagi mountains, it leads to the Wild Monkey Research Institute. They find that a mute custodian named Goro had fed a spider monkey Aoba Walnuts preserved with Aoba Potion. This experimental hormone causes gigantism if taken in high doses. Big "Goro" then comes up with a plan to find his missing friend.

In Tokyo, a huge thorny plant breaks through the streets of Tokyo's business district and sub basements and feeds off of what it catches. It pushes into the roof of the Mainichi Shimpo News offices, where its poisonous flower blooms. Professor Ichinotani believes this plant, Juran, has revived due to man-made environmental changes and seek to destroy it, but Professor Genda wants to study it.