The mysterious Ultraman, Tregear, has suddenly appeared in Ayaka City. Tregear asks what one would do to make their dreams come true, prompting Katsumi to search for his own dream. As a monster appears for the first time in a year, Ultraman Blu and Ultraman Rosso are joined by Ultraman Geed to face off against this new threat. Walking towards their dream with the heart of the Minato family in hand, Ultraman Gruebe is born.
The two main characters, Katsumi and Isami Minato, are brothers who live with their younger sister and father who runs a specialty store. One day, they found the R/B Gyro and R/B Crystals, which transforms Katsumi into Ultraman Rosso and Isami into Ultraman Blu.