Starting with this show, the Ultra Series heads in a notably different direction, slowly shifting from a sci-fi/mystery adventure to full-tilt superhero fantasy fare. All previous Ultramen (Ultraman, Ultra Seven, Ultraman Jack and Zoffy) make regular appearances in this series, whereas they only made guest appearances in the previous show The Return of Ultraman.
Yapool, an alien mastermind with similarities to Satan and Legion, sought after the planet of Earth and tried to conquer it with a campaign that involved his experiments known as Choju. The first super weapon sent to Earth, Verokron, ravaged a city and took on TAC, a special forces unit created by the Self Defense Force to defend the Earth from kaiju. Two of Verokron's unfortunate victims were Seiji Hokuto and Yuko Minami. Before they were dead long the first five Ultra Brothers, Ultraman, Zoffy, Ultra Seven, Ultraman Jack, and Ultraman Ace, decided to revive the duo for trying to help young children get away from the Choju and to fight off Yapool's forces. Ace decided to be the one to go as he had both of them be his host. Yapool has used not only Choju, but also recruited other aliens, humans that sold their souls to him for Choju possession, and even himself to assassinate Ace and TAC only to be brought down in absolute defeat. After the destruction of one of Yapool's oldest Choju, Lunaticks, Minami revealed she was one of the last natives from the moon and gave her Ultra-Ring to Hokuto as she had to leave, but she would return every once in a long while. Ever since Hokuto has been Ace's host and their efforts were just the same. Over the course of the remainder of the series, Ace and the Ultra Brothers proceeded to fight off yet more Choju, including several battles where they were assisted by Father of Ultra, Ace's adoptive father, and Ace's half brother, who was then unnamed. Yapool became desperate by combining the bodies of a selected few to create Jumbo King only for him to fail and have Ace fly off to his home world of M78. Ever since Ace has showed up to help later Ultras in their times of need.