After a fierce battle against Bemular on a desolate moon, Ultraman Mebius returns to his home, the planet Ultra, which is also known as the Land of Light in Nebula M78. Upon his arrival, he joyfully reunites with fellow Ultramen, including Ultraman, Zoffy, and Ultraseven. Meanwhile, on the outskirts of Ultra's orbit lies the Belial Prison, where a cunning alien named Zarab disguises himself as an Imitation Ultraman. He treacherously incapacitates the guards and utilizes the powerful Giga Battlenizer to release the prison's sole inmate—the malevolent Ultraman, known as Ultraman Belial.
Expecting a partnership, Zarab is shockingly killed by Belial, who then retrieves the Giga Battlenizer just as he is confronted by a squadron of Ultras led by Ultraman Taro. Overwhelmingly powerful, Belial swiftly defeats the squadron and continues his rampage, overpowering every remaining Ultra in the Land of Light. This includes his former allies, Ultraman Ken, the Father of Ultra, and Ultrawoman Marie, the Mother of Ultra, as he sets his sinister sights on the Plasma Spark Tower, a crucial source of life for the planet. As the world begins to freeze due to Belial's onslaught, Ultraman and Ultraseven make a desperate stand to save what's left of their home, even as Taro tries to protect the last vestige of hope against utter destruction.
In the aftermath of Belial's chaos, Mebius returns to a tragic and frozen wasteland, guided by Ultraman and Ultraseven to seek out Rei, who possesses the key to their salvation. Following their previous encounter with Alien Reiblood, the ZAP Spacy group arrives on Dent, a planet resembling Earth, where Rei and his companion, Gomora, have just vanquished a Zaragas. In a remarkable twist, Mebius appears before Rei in his human form, Mirai Hibino. Mirai explains the origins of Planet Ultra and the vital role of the Plasma Spark—created to replace the dying sun of M78, it allowed Ultramen to flourish as protectors against Kaijus, birthed from the very radiation that also elevated Ultramen. However, Belial presents a grim exception, as his unquenchable thirst for power prompts his banishment after nearly wrecking their world.
Having allied with Alien Reiblood, Belial further descends into darkness, becoming an Ultraman-Reionics. His powers enable him to harness and control countless Kaijus with the Giga Battlenizer, leading to the disaster known as the Belial Revolt, where he almost conquered the Land of Light before being confined to the newly constructed Space Prison by Ultraman King. With Zarab’s deceit, he has now escaped and plots to resurrect his formidable army with the power of the Plasma Spark, and Rei ultimately agrees to aid in their fight against this formidable foe.
Rei and Mirai embark toward the Plasma Spark Tower on the now-frozen surfaces of Ultra but are thwarted in their path by an ambush from Alien Shaplay, who is accompanied by minions—a Dorako, a Bemstar, and a Salamandora—sent by Belial. During this fierce clash, Mirai's Mebium Brace becomes compromised by the Shaplay, putting him at a disadvantage. Fortunately, Ultraman and Ultraseven, disguising themselves as human forms, intervene just in time to assist. They bring forth their Capsule Kaijus, including Windom, Miclas, and Agira, successfully overcoming the monstrous threats. As their journey progresses, they learn of another Ultraman who could potentially help, but Ultraseven discerns this individual was also exiled from the Land of Light.
Upon arriving at the tower, they encounter a vengeance-fueled Shaplay while Rei and Gomora face the deadly Black King. After overcoming these obstacles, Ultraman Taro unites their remaining strength, channeling the last of the Plasma Spark's light to empower them into their original Ultraman forms in a desperate final effort to reclaim the Plasma Spark from Belial. Meanwhile, on a distant desert planet, another Ultraman, known as Zero, undertakes vital training under the watchful eye of Ultraman Leo while Ultraman King observes.
As the battle heightens at the Kaiju Graveyard, the Ultramen, along with Rei's Kaijus, confront Belial's monstrous army. During this tumultuous encounter, Belial feels a strange kinship with Rei, attempting to sway him to join his dark cause. When Rei steadfastly refuses, Belial uses his powers to forcefully manipulate the human into his Burst Mode, leaving him perilously under Belial's control. Not to be outdone, Gomora transforms into Reionic Burst Mode, attacking the Ultramen while Ultraseven suffers grievous injuries. Meanwhile, the ZAP Spacy crew strive to locate Rei, only to be ambushed—but they are fortuitously rescued by Ultraman Dyna, who pledges to assist them in reaching the Kaiju Graveyard after acquiring information about Rei's whereabouts.
In an intense showdown, Ultraman Dyna engages Belial, while the ZAP Spacy crew successfully pull Rei from the throes of his Burst Mode. Belial, refusing to relent, aims to eliminate the humans but suffers pushback when Ultraseven heroically steps in to absorb a fatal blow. Administering the last vestiges of his power, Ultraseven dispatches his Eye Slugger to the desert planet just as Ultraman Zero successfully completes his training by rescuing a Pigmon in distress. Both Leo and Ultraman King illuminate the truth to Zero regarding his lineage, revealing that Ultraseven is his father and guiding him to avoid the same destructive path chosen by Belial.
Upon grasping this profound truth, Zero discards his training gear and races toward the battle at the Kaiju Graveyard, introducing himself as Ultraman Zero. In a heart-wrenching moment, as Ultraseven falls in battle, a furious Zero unleashes his full might upon Belial, vanquishing the Kaiju army before confronting the evil Ultraman directly. The clash that follows is monumental, culminating in Zero disarming Belial of the Giga Battlenizer and sending him plummeting into a river of molten lava. However, the spirits of Belial's defeated Kaijus amalgamate into a colossal exosuit-like creature dubbed Belyudra, overpowering the Ultramen once more.
Just when all hope seems lost, Rei channels the Giga Battlenizer, disrupting the connection that binds Belial to the Kaijus. This diversion opens the way for Zero to harness energy from the restored Plasma Spark, empowering him to finally bring an end to Belial's reign of terror as he is engulfed by Belyudra in a cataclysmic explosion. With Belial defeated and the Plasma Spark reinstated to its rightful location, the ZAP Spacy team achieve a remarkable milestone as they set foot on the Land of Light, bidding farewell to Ultraman and Mebius, while Dyna decides to accompany them for a while longer in his human form, Shin Asuka. The remaining Ultras gather to listen to a stirring address from Ultraman King, who emphasizes the necessity to remain vigilant against impending threats to their universe. Meanwhile, an unexpected revelation emerges from the ashes, as a scarred Belial is unveiled, having survived the destruction of Belyudra, hinting that the battle between good and evil is far from over.
