From another dimension, Yapool orchestrated attacks on Earth using biological weapons called "Terrible-Monsters" (超獣, Chōjū) that surpassed ordinary monsters.

A pair of youths, Seiji Hokuto and Yuko Minami, lost their lives in protecting nearby civilians as the fifth Ultra from M78, Ultraman Ace fused with them.

In the middle of the series, Yuko was revealed to be one of the last natives from the Moon and gave her Ultra Ring to Seiji, leaving Earth in episode 28.

When Yapool allowed himself to be killed as the children lost their faith in Seiji, the latter exposed his identity to them and fought the chimeric monster, eventually triumphing over Jumbo King.

Seiji Hokuto, a bakery delivery man, and a nurse named Yuko Minami are killed in the destruction while trying to help fleeing citizens.

Seiji and Yuko become members of TAC (Terrible-monster Attacking Crew), a Self Defense Force funded by the global combat organization tasked with battling the Yapoolian threat and their army of inter-dimensional monstrosities.