In Planet Sundowin, Ultraman X and Geed were sent to apprehend Dark-killer after he terrorized the universe until their energies were stolen for the creation of X and Geed Darkness. The two copies defeated their counterparts and proceed to attack Orb on Planet O-50. Due to Zero’s failure in protecting Grigio and ended up within the Dark-Killer Zone, the Ultra Brothers summon Rosso and Blu to their headquarters but they fly away before Taro can provide enough explanation, leading to their encounter with Etelgar. After saving a lone Ragon, Ribut bailed Rosso and Blu from Planet Penol whereas Victory and Ginga rescued Orb, X and Geed respectively.
Taro explained Dark-Killer’s origin: a being of darkness born from the grudges of past monsters that the Ultra Brothers fought before. Taro was able to defeat him with the Ultra Brothers’ help, but Dark-Killer was revived, conscripting the resurrected Dark Lugiel and Etelgar to the League of Darkness as he create armies of evil Ultras for intergalactic conquest. Using Zero and Grigio as hostages, Dark-Killer goaded the New Generation Heroes to Planet Tenebris, which they did so despite the possibility of getting into a trap. Orb, Geed and X stay behind to fight their copies, Ginga and Victory faced Lugiel and Etelgar in the Dark Palace as Rosso and Blu entered the Dark-Killer Zone, only to be ambushed by the namesake villain himself. Meanwhile, Ribut unmasked Tregear as the mysterious benefactor of the League of Darkness and attempted to arrest him. Under memories of Ultraman Zero, X, Orb and Geed assumed their Zero-related forms to eliminate their copies while Ginga and Victory merged to destroy their revived rivals. Grigio managed to muster enough strength to get pass Zero Darkness to recharge her brothers, giving them a chance of counterattack by forming Ultraman R/B and killing both Zero Darkness and Ultra Dark-Killer.
In an attempt to gain advantage against the New Generation Heroes, Tregear revived Zero Darkness and Dark-Killer (the latter enlarged to 200 meters) before escaping from Ribut and send the Red Kings as his distraction. After transforming Zero into Zero Beyond for his fight with Zero Darkness, the New Generation Heroes accessed their strongest forms for the final battle. The insane Dark-Killer made his desperate attempt by recreating the evil Ultramen to consume them and grew larger. Under Taro’s instructions, the New Generation Heroes charged the Strium Brace for Ginga Strium to perform New Generation Dynamite. While Dark-Killer’s fate was guaranteed not to return, Zero revealed that Grigio’s kidnapping was to prevent the formation of Groob. The only person other than the Ultras to know about this is none other than Tregear, who voiced his intent to attack the Land of Light and left immediately. The rest of the New Generation Heroes chased after the rogue Ultra as Zero sent Grigio back to her home world, wanting her to protect Ayaka City in her brothers’ absence.