A few months have passed since Seijuu Sentai Gingaman defeated the Space Pirates Balban and saved Earth. Now, Ryouma and his comrades are back home in the Ginga Forest resuming their peaceful lives, while their partner Starbeasts, who went on a journey spanning the universe, are expected to return to them soon. The Gingamen are enjoying the tranquility of their home, but Yuuta and Fairy Bokku are wondering why the Starbeasts are late. All of a sudden, the Gingamen's mentor, Tree of Wisdom Moak, suddenly senses an unknown object heading towards the city. This startling announcement prompts Ryouma to bring his team to investigate the thing.
In the city, the Gingamen witness a huge red monster laying waste to the buildings and transform to try and defeat it. However, out from the beast emerges a mysterious humanoid alien who introduces himself as the Gelmadix's captain, Space Pirate Gregory. The Gingamen try to engage him, only for Gregory to prove very formidable until he is interrupted by a shot from somewhere. That shot came from Mega Red, who proves to be a valuable ally to help keep Gregory busy. When the Space Pirate commands his monster, Ghelmadix, to destroy the six heroes, Giga Bitus enters the fray and deploys both Giga Phoenix and Giga Rhinos, but the two Steel Starbeasts end up getting destroyed, after which, Gregory withdraws from the battle while the beast also makes its leave into the Earth.
Hiding his fortress away on a remote island, Gregory notes that his trusty monster has been injured when his assistant, Hizumina, approaches him, suggesting that they erect a barrier around Earth to prevent the Starbeasts from meddling with their plan. Hizumina then explains that Gregory's sword has drained its energy from battling the Gingamen and Mega Red, suggesting that she should be sent to find another source of energy to re-power it. However, Hizumina is still very focused on reviving her faction, the Nezirejia, which she also brings up to Gregory, who promises to bring them back. As for Gregory, he uses whatever energy he has left in his sword for a ritual.
Mega Red, also known as Kenta Date, is shown around the Ginga Forest, where he meets Moak. When the Gingamen question Moak about their enemy, Moak explains that there was once a pirate who was sealed away in space by the Starbeasts, but he doesn't know where he or his monster is. Kenta offers to help the Gingamen find them both, approaching Chisato Jougasaki/Mega Yellow at Meihoh University for the job. Chisato, who was also working on the case, asks Ryouma for his Ginga Brace for analysis, complaining about her organisation's radar system being blocked by a radar-jamming barrier. As Kenta and Ryouma discuss about their duties after completing their missions as warriors, a mysterious woman steals the Ginga Brace Chisato was working on.
Kenta and the Gingamen pursue her into the forest, where the woman reveals herself to be Hizumina, Doctor Hinelar's last android with similar looks to Shibolena, seeking to revive the Nezirejia and defeat the Megarangers. Gregory also reveals himself, stating that he aims to revive all of Balban to destroy the Gingamen. Ryouma attempts to reclaim his brace, but is wounded, forcing his comrades to try to help him. However, the revived generals of Balban suddenly corner the team, demanding they hand over their braces as well. Suddenly, Ryouma's brother, Hyuuga, and his own Starbeast, BullTaurus barge in to save Ryouma, forcing the villains to retreat.
Ryouma lies recovering in a medical ward, but is still determined to reclaim his Ginga Brace, which Gregory inserts into his sword's hilt to revive Captain Zahab. Despite interference from Hizumina who tries to steal the sword to revive the Nezirejia, the ritual goes smoothly and Captain Zahab is resurrected. Zahab thanks his old friend and is excited when he tasks his generals to give Ghelmadix more power, intending to use it as his and Gregory's new ship to resume plundering the universe once again.
Meanwhile, Chisato, Kenta and the remaining Gingamen worry over finding Gregory, when the other three Megarangers approach them with a device to help them locate Gregory's fortress. As the others leave to find the fortress, Kenta approaches a still recovering Ryouma, knowing his desire to reclaim his brace and offering him a ride. However, on the way there, Kenta and Ryouma are thrown off Kenta's motorbike due to the tremors from Ghelmadix. As for their respective teams, they hop on a boat and navigate their way to Gregory's island, facing off with the Yartots. However, when they try to engage Gregory and Zahab while they transfer the energy from Gregory's sword to the beast, Zahab's generals ambush them. Suddenly, Kenta and Ryouma, swooping in on Kenta's Cyber Slider, attack the two captains and reclaim Ryouma's Ginga Brace. Angered, Gregory throws a bunch of Balban's monsters at the Megarangers and the Gingamen, only for the horde to fall before the might of the two teams.
As the Gingamen confront Zahab, Gregory and their crew, a badly damaged Hizumina engages the Megarangers, but is gravely wounded by Kenta. Disregarding her injuries, she attempts to fall on Kenta to take him out as she dies, but only succeeds in momentarily battering the All-Purpose Hero. Just then, Ghelmadix emerges from the Earth more powerful than before, forcing the Megarangers to call on their shuttle to form Galaxy Mega and fight it. Gregory also enlarges to help his monster defeat their enemies, forcing Hyuuga to combine with BullTaurus to aid Galaxy Mega. As the battle rages on, the Megarangers receive a message from INET: they were sending them a newly-invented satellite which would enhance the Gingamen's powers to their location. Upon hearing word from their new allies, the Megarangers give the news to the Gingamen, instructing them to fire their Earth Powers at the satellite. Despite interference from Zahab and his crew, the Gingamen defeat their enemies and follow the Megarangers' instructions, blowing Gregory's fortress to smithereens. Infuriated by the loss of his comrades and his fortress, Gregory attempts to retaliate, but the Starbeasts intervene, allowing the Gingamen to join the other two in their robot battle and destroy both Ghelmadix and Gregory.
Afterwards, the Megarangers pay a visit to the Ginga Forest and socialise with their new friends. Ryouma and Kenta ponder over the beauty of the forest and the amazing wonders of technology, with Kenta musing he would have to visit the forest on special occasions.
