The SDF second lieutenant, Takumi Hidemoto, who is investigating the unidentified flying object that fell in the Pacific Ocean, encounters a sudden blue light. Affected by the luminous object, his genetic factors undergo mutation, transforming him into the fierce monster THEONE. Afterward, he (THEONE) escapes from the surveillance of the special agency BCST within the Ministry of Defense and is never heard from again. Meanwhile, Air Self-Defense Force pilot Shunichi Maki (Tetsuya Bessho), decides to resign in order to spend more time with his son, Kiyomune, who has a congenital disease. During his last flight, Maki encounters a 'red luminous body' in mid-air. This occurs three months after the THEONE incident. However, Maki miraculously survives and transitions to becoming a commercial pilot, starting a new life. The agency is particularly interested in Maki, especially the chemical officer Sarasa Mizuhara from BCST, who forcibly takes him away for physical monitoring in a makeshift BCST laboratory set up in an abandoned factory. At this moment, the missing monster THEONE appears to be drawn back by something. After being attacked by THEONE and on the brink of death, Maki's body undergoes a transformation, turning into a giant form blessed with a 10-meter tall steel body that shines silver, occasionally retaining his own will. He engages in battle with THEONE, but due to incomplete transformation, his energy is quickly drained, and after receiving a decisive blow, he reverts back to Shunichi Maki. Sarasa then names this newly emerged life form 'THENEXT.' On the way back to the BCST laboratory from Tokyo, Maki learns that his son Kiyomune has been hospitalized. Upon arriving at the hospital, he meets his wife, Ryoko, once again. At this moment, humanity faces its greatest crisis as the mutated 'THEONE,' having absorbed various organisms into its body, resurfaces on the streets of Shinjuku. The overwhelming fireballs shooting from its mouth devastate the streets, and the person known as 'THENEXT' has been replaced by 'THEONE,' which has become incapable of returning to human form. Nevertheless, the former couple, who were previously betrothed, have memories held in Sarasa's heart, and for the sake of his promise to his son Kiyomune and to protect his beloved family, Maki transforms once again. This time, his form evolves into a gigantic 40-meter behemoth, with red-like bleeding at the joints, and elbow blades emerging at his elbows. Now, in the sub-center of Shinjuku, the battle between ULTRAMAN and 'THEONE' ignites!
