In the Amagi mountains, zoologists investigate the appearance of a colossal monkey reported to be jovially swinging from ropeway cables near a hot springs resort. The trail leads to the Wild Monkey Research Institute, where a mute custodian, Goro, had befriended and fed a spider monkey from a store of Aoba Walnuts at the facility. Unbeknownst to Goro, they were preserved with Aoba Potion, and experimental hormone supplement, which causes thyroid imbalance and gigantism in high doses. Big "Goro" comes to the city to look for his missing friend--and a plan is put into motion.
