WARNING: Contains nothing but spoilers
The film begins with two orbs of energy zooming through space, a red one in pursuit of a blue one. These two orbs crash on a desert planet and they reveal themselves to be Ultraman Mebius and the monster Bemular respectively. After a brief battle which ends in Bemular's defeat, Mebius heads back to M78. After a sequence telling the story of the Showa Ultraman universenote , the viewers are returned to Mebius arriving back at the Land of Light. Soon, Mebius meets up with Ultraman, Zoffy, and Ultraseven. He informs them of his completed mission and his superiors note that monster rampages have grown more frequent around the universe due to a growth in "minus energy" indicating something very dreadful is about to happen.
Sure enough, miles above the Land of Light's surface, an ultraman wielding the Giga-Battlenizer, a device that allows the user to control up to 100 monsters and aliens, arrives at a strange cube-like structure, referred as the Space Prison, that acts as the planet's moon. The hero defeat the two guards outside the prison's gates and enters. The intruder unveils himself as the shapeshifting Alien Zarab and marches to the one thing in the prison - a being partially imprisoned inside a wall in a state of suspended animation. Zarab inserts the Giga-Battlenizer into the prison's lock and it opens. The being steps out and Zarab is overjoyed to have finally released the dark warrior of the Land of Light - Ultraman Belial. Zarab begs Belial to form an alliance with him and rule the universe with the Giga-Battlenizer, but Belial kills the alien in cold blood. That is when Ultraman Taro arrives at the Space Prison with a huge squad of ultra warriors behind him. Belial singlehandedly defeats the entire squad before taking on Taro. The two plummet to the Land of Light below in the battle and Taro is swatted away by Belial.
The citizens of M78, now aware of Belial's return, all take on the dark ultraman, but, once again, Belial overpowers every challenger. He begins to make his way to the Plasma Spark Tower, the building that holds the Plasma Spark, the source of the ultramen's power and the planet's light. He is faced off by Ultraman, Seven, Mebius, Zoffy, Ultraman Jack, Ultraman Ace, and Ultraman 80. They too are all beaten. Belial reaches the Plasma Spark's chamber and is confronted by the Ultra Father, the Ultra Mother, and Taro. Belial defeats them all as well and succeeds in taking the Plasma Spark. The Land of Light begins to freeze over, encasing every inch of it and every ultraman (with the exceptions of Ultraman, Seven, and Mebius as the former two managed to set up a shield just in time while the latter was thrown into the safety of space by Belial) in vast amounts of ice. Ultraman Belial then leaves M78 for the Monster Graveyard, the place where all monsters and aliens slain by ultramen go. However, Taro manages to cover up a tiny piece of the light left in the chamber, keeping the Land of Light just barely alive. Meanwhile, Mebius meets up with Ultraman and Seven on the frozen waste. The older heroes instruct Mebius to find the human Reionics known as Rei, a member of the the spacefaring Earth organization known as ZAP SPACY. With that, Mebius takes flight in search of Rei.
Elsewhere in space, ZAP SPACY have arrived on the Earth-like planet of Dent. Their peace is interrupted by the sudden appearance of the monster Zaragas. Rei unleashes his Reionics monster Gomora and the two monsters battle it out. Gomora eventually destroys Zaragas with its Super Oscillatory Wave, but before ZAP SPACY can get back to what they were doing, Mebius arrives. The ultraman takes Rei without warning and flies off, leaving ZAP SPACY confused. At the Land of Light, Rei meets Mirai Hibino, the human form of Mebius. Mirai explains to Rei that the ultramen need his assistance. He proceeds to explain to him (and the audience) the full story.
Long before there were Ultramen, a terrible alien being named Rayblood existed. He terrorized the universe with his ability to command monsters. Elsewhere in the universe, M78 was inhabited by a race that resembled humans in every manner. 270 000 years ago, their planet's sun died and these people created the Plasma Spark as a replacement. However, the power of the Spark's light transformed them into the Ultramen. One of these new beings sought to take the power of the Plasma Spark for himself and was exiled for it. Wandering the universe along, the young ultra warrior encountered Rayblood. Rayblood gave the exile the power he sought by possessing the fallen hero, transforming him into Ultraman Belial, the universe's first Reionics. Belial created the Giga-Battlenizer and used his newfound powers to attack the Land of Light with an army of monsters and evil aliens. He seemed unstoppable until the mighty Ultraman King appeared and created the Space Prison to punish Belial. With Belial defeated, the monsters were subdued and King placed the Giga-Battlenizer in the fiery caverns beneath the Monster Graveyard's single dark landmass note .
With the story explained and Rei accepting the honour of saving the Ultramen, Mirai and Rei begin to make their way to the Spark Tower. Elsewhere, Belial has reached the Monster Graveyard and resurrected many monsters, using the power of the Plasma Spark and the Giga-Battlenizer. He prepares to eliminate any surviving Ultramen and seize the universe. Mirai and Rei become targets of Belial's hunt when the monsters Dorako, Bemstar, and Salamandoras attack them along with Alien Shaplay, all sent by Belial. Rei is unable to call forth Gomora when Shaplay manages to separate him from his Battlenizer and Mirai loses his transformation into Mebius thanks to Shaplay damaging his Transformation Trinket. That's when Shin Hayata and Dan Moroboshi, the respective human forms of Ultraman and Seven, arrive. Dan calls forth his Capsule Monsters Miclas, Windam, and Agira to battle Shaplay's monsters and Hayata manages to bury Shaplay under a pile of icy rubble with a handy machine gun. With the monsters defeated, the group makes their way.
Dan explains to Rei that Taro has the rest of the light and it carries the necessary energy needed for Hayata, Dan, and Mirai to return to their Ultraman forms. Mirai suggests they call in a mysterious warrior, but Dan refuses as he feels this warrior, while powerful, is foolish and brash, unable to understand that to have great power, one must have great justice and control. This warrior tried to take the Plasma Spark as well and almost went the way of Belial, thus was banished by Seven and sent far away from M78 to train under Ultraman Leo. Meanwhile on desolate K76, Leo trains this mystery hero (covered by disabling Techtor Gear) through fierce combat, in which the warrior is easily beaten in.
Rei, Mirai, Dan, and Hayata finally reach the Plasma Spark Tower, but Shaplay returns, revealing himself to be a Reionics in command of the herculean power of Black King. He sends his monster partner to attack, but Rei counters with Gomora. While Rei and Gomora keep Black King busy, the ultramen in human form enter the Plasma Spark Tower. and find Taro's frozen body with the light to his chest. Shaplay comes after them, but Dan's Ultra Psychokinesis and a hail of bullets from Hayata kill the alien. Gomora destroys Black King outside the tower with a Super Oscillatory Wave. Inside the tower, Rei regroups with Mirai, Dan, and Hayata. The ultramen in human form regain usage of their transformation devices from Taro's light and return to their usual selves. The three take Rei to where they correctly believe Belial is - the Monster Graveyard.
At the Monster Graveyard, Ultraman, Seven, Mebius, Rei, and Rei's monsters, Gomora and Litra face Belial. Belial takes interest in Rei for also being a Reionics like him and summons his army of monsters and aliens to defeat the heroes. The monsters and heroes battle, and while Ultraman, Seven, Mebius, and Gomora are able to kill many monsters, the sheer numbers make the battle long and difficult.
Back at Dent, ZAP SPACY is over the planet's surface in their ship, the Pendragon debating over whether they should go to M78 in pursuit of Rei and Mebius. However, the Pendragon is attacked by the robotic dragon Narse while Alien Zetton (aka Zettonian) boards the ship, both minions of Belial sent from the Monster Graveyard to stop ZAP SPACY from joining the battle. That's when a mysterious man appears to fight the Zettonian. He kills the alien and reveals himself to be Shin Asuka, or Ultraman Dyna. He agrees to take ZAP SPACY to the Monster Graveyard. Dyna turns back to his real form and destroys Narse before transporting ZAP SPACY.
Back at the Graveyard and the main battle, Belial notices Rei become his Super Mode Reimon to defeat some alien minions (which he does successfully) and gets an idea. He knocks down Mebius, Seven, and Ultraman with some blasts of energy before physically overpowering Gomora and Litra. With all heroes of his size out of the picture, Belial taps into his Rayblood self, using it to turn Rei into his Reionics Burst Mode. Gomora follows suite and Belial has his surviving monsters fall back in hopes that Reionics Burst Gomora' brutality is sufficient to win. The ultra heroes' attempt to subdue Gomora fail, but Dyna and ZAP SPACY arrive in time. Dyna fights Belial and is defeated while ZAP SPACY manage to free Rei (and consequently Gomora) from Reionics Burst. Enraged, Belial tries to kill the humans, but Seven leaps in front of the blast of energy, deeply wounding the hero. In his dying moments, Seven throws his Eye Slugger to send a message of K76.
On K76, Leo's battle with the mystery warrior sees the latter stop a huge boulder caused by their battle from crushing the tiny, friendly monster Pigmon. Leo is pleased to see that the hero has learned the meaning of being an ultraman, and he, with King and Astra, announce that his training is complete. That's when the Eye Slugger arrives. King uses his powers to read Seven's intended message and the three ultramen send the hero to defeat Belial, revealing to the audience that he is Ultraman Zero, the son of Seven! Zero is freed from his Techtor Gear and soon arrives at the Monster Graveyard.
At the Graveyard, Belial is ready to finish off the heroes when Zero arrives, just in time to see his father's eyes darken. He turns to Belial and manages to singlehandedly defeat all the remaining monsters with no problem before taking on the dark ultraman. After a lengthy fight, Zero disarms the Giga-Battlenizer from Belial's hands and tosses the villain into a ravine of lava. Leo and Astra arrive and everyone gets ready to take the Plasma Spark home, but suddenly Belial and all his monsters (and all the other souls in the Graveyard) return as Beryudora, a monster made out of monsters. Ultraman, Seven, Leo, Astra, Dyna, Mebius, Gomora, the Pendragon, and Zero all take on the colossus. Rei uses the Giga-Battlenizer with his to communicate with the monsters and manages to get them revolt against Belial, disabling Beryudora. Zero then turns to the Plasma Spark, which willingly hands its power to him, and gains a great power to defeat Beryudora. All the other heroes follow up with their abilities and Beryudora is destroyed along with the Giga-Battlenizer.
At the Land of Light, the Plasma Spark is returned to its rightful place. With that, the Land of Light is revived with all its inhabitants and Seven is brought back and reunited with his son. ZAP SPACY become the first humans to set foot on the Land of Light and the ultramen give their gratitude for helping in save the universe. As ZAP SPACY leave M78 and Dyna heads off to other places, King gives a speech to the people of M78. In some distant part of the universe, Belial's body lies lifeless...but then his eyes glow once more. But, the consequences of that are a story for another time.