Ultra Galaxy Legend Gaiden: Ultraman Zero vs. Darklops Zero (ウルトラ銀河伝説外伝 ウルトラマンゼロ VS ダークロプスゼロ Urutora Ginga Densetsu Gaiden: Urutoraman Zero VS Dākuropusu Zero) is a Japanese direct-to-video two part series that was made as a prequel to Ultraman Zero The Movie: Super Deciding Fight! The Belial Galactic Empire.
Stage 1: Cosmic Collision
Zero sits on a cliff while he watches a strange tower firing lightning bolts, and then rises to face against what seemed to be members of the Ultra Brothers: Ultraman, Zoffy, Ultraman Jack and Ultraman Ace. After incapacitating the ZAP SPACY's Space Pendragon, Ultraseven joined the group as they faced against Zero.
Meanwhile, Rei and Hyuga sensed a distress signal from a nearby ZAP SPACY base. They dispatched on the Space Pendragon and track the signal's source from a recent space distortion, which successfully dragged their ship into it. After awakened, they find themselves within a subspace linked to multiple distortions. The distress signal from before triggers again but the source is revealed to be from another Space Pendragon within a strange planet. As they explore it, they spotted Ultraman Zero and several other Ultra Warriors while being chased by two mysterious objects before the Pendragon crash-landed. The two set on foot and track the source of the signal, which was revealed to be from a wrecked Space Pendragon and found an injured Kumano, as well as another Hyuga, revealing them to be the source of the signal. As Kumano disappears, the second Hyuga revealed that since the past two weeks, time distortions begin to appear out of nowhere. The alternate ZAP SPACY arrives on a planet and fights alongside Ultraman Zero but has reached their limit.
After witnessing on what appears to be corpses of Ultraman Jack and Zoffy, the alternate Hyuga reveals that a strange force that resided in the factory they saw performs an experiment which causes multiple distortions to different universes appear. The alternate ZAP SPACY (Haruna and Oki included) slowly disappears due to their existence being abnormal, as the planet they stepped on becomes one that doesn't belong to any plane of existence. The facility produces nigh-unlimited supplies of fake Ultra Brothers, with their opponent Zero shall met his limit in any moment. In his final breath, the alternate Hyuga orders the prime ZAP SPACY to combine forces with Zero, destroying the facility and save the multiverse before disappearing.
Not long after, several warheads target Rei and Hyuga until Gomora shields them. The perpetrator reveals itself as a robotic copy of the same monster and managed to defeat the original Gomora. The perpetrator watches as she revealed that the robotic Gomora is built based on the original template, making it twice powerful than the original. Before the robot can finish off its template, Zero appeared and switch places with an injured Gomora to fight the fiend, giving Rei an opportunity to attack the alien factory with Litra. Seeing how Zero being able to defeat the mechanical Gomora, the alien leader Herodia sent the experimental robot equipped with their Techtor Gear.
Before Zero can finish the robotic monster, he was interrupted by a strange mechanical figure and prompts to fight it. Rei tries to attack the alien factory but was shielded by a barrier and caught in an attack by the battle-damaged mechanical Gomora. Hyuga tries to help but gets caught in a crossfire between Zero and the Techtor Gear warrior. The mechanical warrior destroys his Techtor Gear by his own will and reveal himself as Darklops Zero and defeated his original template. Reinforcements arrived in the form of mechanical copies of Ultraman, Seven and Ace while Darklops banishes Zero and the impostor Ultra Brothers to a different subspace.
Stage 2: Zero's Suicide Zone
Meanwhile, Zero finds himself trapped within a universal gap and faced against an impostor Ace. Following the battle, Darklops Zero was restrained once more by Herodia, remembering a figure named "Kaiser Belial" before deactivating. Rei was restrained after his failed attempt to intrude the alien base but was rescued by none other than his alternate self. Unlike the prime Rei, alternate Reimon was never able to control his powers, forced into the state of Reionic Burst until the original Rei punched Reimon, thus deactivating its Reionic Burst mode. Rei revealed that he lost his Rayblood instinct after Hyuga threw Seven's Eye Slugger, creating a scar on his chest which symbolizes the former's friendship with ZAP SPACY members. While searching for Rei, Hyuga instead finds his Neo Battle Nizer and an entrance to the alien base made by Litra. Although the fake Ace was destroyed, Zero try to find an escaped but was halted by Seven and Ultraman impostors.
Arriving at the launch facility, Rei and alternate Reimon discover an army of mass-produced Ultra Brothers and met Herodia, the Alien Salome that also stole Reimon's Battle Nizer to create Mecha Gomora. Despite being unstable, but Darklops Zero's Dimension Core is successfully held in the Alien Salome's control, allowing them to sent an army of fake Ultra Brothers to various universes. With Zero at his limits, Leo arrives to his aid while reminding his student of the harsh training he previously faced. Hyuga appears and gives Rei the Neo Battle Nizer, summoning Gomora to stop the Salome's plans while a repaired Mecha Gomora was sent to fight the Battle Nizer monster as well. Herodia tries to get her servants to work but was captured by Darklops Zero while her servants died from their base's rubble.
Darklops Zero proceeds to destroy the entire factory and reprimand his captor of how living organisms being selfish before she fell from a great height. In order to escape the universal gap, Leo told his student to re-enact his and Astra's Ultra Double Flasher, which they succeeded and at the same time destroying the fake Ultraman and Seven. In her final breath, Herodia revealed to Hyuga that Darklops Zero was a creation of unknown force, as they only salvaged the robot when it was shown floating in outer space.
Darklops Zero marches towards the Gomoras' conflict, and successfully reprograms the rogue Mecha Gomora to his servitude as they overpower the original Gomora until Zero arrives. Seeing how Mecha Gomora being a product of his Battle Nizer, alternate Reimon proceed to help Rei controlling his Gomora while Zero arrives just in time to assist the monster against their doppelgängers. After Mecha Gomora's destruction, the alternate Reimon begins to disappear but because of their combined control of Gomora, memories of the original Rei was transferred to the alternate counterpart. This allows him to finally be free from Rayblood's instincts, thanking his prime counterpart before vanishing. With Darklops Zero at the verge of defeat, he reminds his counterpart that the actual threat shall appear from his own home world and prepares to self-destruct. Rei, Hyuga, and Zero tries to escape as Planet Chain and the entire subspace caught in a huge explosion.
After awakened, Rei and Hyuga found that they returned to their original universe and received a message from their original counterpart, having revived after Darklops Zero's destruction. As the Space Pendragon left, Zero saw them from behind and join his master.
- Alien Salome
- Imitation Ultra Brothers
- Darklops Zero
- Mecha Gomora
- Rei (レイ Rei)/Alternate Rei (代替 レイ Daitai Rei): Shota Minami (南 翔太 Minami Shota)
- Hyuga (ヒュウガ Hyuga)/Alternate Hyuga (代替 ヒュウガ Daitai Hyuga): Konishi Hiroyuki (小西 博之 Konishi Hiroyuki)
- Alternate Kumano (代替 クマノ Daitai Kumano): Mitsutoshi Shundo (俊藤 光利 Shundo Mitsutoshi)
- Alien Salome "Herodia" (サロメ 星人 ヘロディア Sarome Seijin Herodia): Tomomi Miyashita (宮下 ともみ Miyashita Tomomi)
- Alien Salome "Irate" (サロメ 星人 イラテ Sarome Seijin Irate): Daisuke Terai (寺井 大介 Terai Daisuke)
- Alien Salome "Ganaesu" (サロメ 星人 ガナエス Sarome Seijin Ganaesu): Hiroshi Suenaga (末永 博志 Suenaga Hiroshi)
- Ultraman Zero (ウルトラマンゼロ Urutoraman Zero)/Darklops Zero (ダークロプスゼロ Dākuropusu Zero): Mamoru Miyano (宮野真守 Miyano Mamoru)
- Ultraman Leo (ウルトラマンレオ Urutoraman Reo): Ryū Manatsu (真夏竜 Manatsu Ryū)