The story takes place in a small city called Xingyuan City. On the Lion's Tail Mountain in the city, there stands a massive object shining with colorful light, known as the 'Horn of Mono.' This object has actually been there since the 'Monster Day' incident that occurred 16 years ago, which was a day when monsters appeared all over the world simultaneously. After the 'Monster Day,' monster disasters began to become a part of everyday life. While Japan's Earth Defense Force began to deal with monsters using military force, the Kaiju Disaster Prevention Scientific Investigation Agency (collectively known as SKIP) continuously conducted scientific investigations and provided evacuation guidance to prevent monster disasters and reduce the damage. To this day, this organization has been investigating the reasons for the appearance of 'Monster Day' and the 'Horn of Mono' in the city. Seven-year-old 'Yuma,' while camping with his parents on Lion's Tail Mountain, encountered a monster attack; miraculously, he survived unharmed. This experience prompted Yuma to embark on a journey to study monster biology. Despite his painful past, Yuma did not give up on his dreams; he grew up relying on the 'power of imagination' and joined 'SKIP' as a rookie investigator, assigned to the branch in Xingyuan City. At this moment, a large-scale monster disaster struck Xingyuan City. "I want to protect these desperate people right in front of me!" This strong and direct thought surged from the depths of his heart, and in that instant, the light messenger 'Ruthion,' whom he had seen in his childhood, appeared in Yuma's mind and spoke to him: "I am you, and you are me... unleash your imagination!" A mysterious light appeared in his hands, enveloping Yuma's body, and the power of imagination combined light with humans, transforming into the light giant to protect the future, 'Ultrakidon!' Together with his important companions, Yuma and Ultrakidon chase their ever-unending dreams and begin to run from this moment on!