Flashmen are the main protagonists in Choushinsei (supernova) Flashman.
They are originally a group of five children from Earth who were kidnapped by the an alien group known as the Alien Hunters, for the sake of the Reconstructive Experiment Empire Mess - who wanted samples of humans from Earth to experiment on. They were rescued by the Flash alien race, which took each one to a different planet of the Flash solar system for training. Each of them was trained separately from the others in a range of superpower abilities that will allow them to fight Mess. When they finally return to Earth in 1986 to combat Mess, who is now trying to invade it, they use the opportunity to search for their birth parents. They later learn that their time on earth will be cut short. Their birth planet's atmosphere has become deadly to them and after a year, they will die from what is known as the Anti-Flash Phenomenon.
Anti-Flash Phenomenon is a potentially dangerous allergic reaction to environments that are from non-Planet Flash. It occurs to anybody who had lived in planet Flash for a long time leaves to dwell on other planets. It was due to this phenomenon that Flashmen's alien mentors were initially hesitant to send them to Earth. The only cure for the anti-Flash phenomenon from claiming lives of Flash residents is for Flash residents to return to their homeworld.
At first, the phenomenon would trigger only a minor physical discomfort, but as the story progresses Flashmen would have problems contacting sunlight, physical contact with other Earthlings, Earth's plants etc. Eventually in later episodes the phenomenon had gotten progressively worse to the point that Flashmen could no longer consume or drink foods made from Earth - signalling the time Flashmen can spend on Earth from that point on would be limited to mere days. If Flashmen go pass beyond the said limited days, ultimately they would not be able to breathe Earth's air and they would die. Fortunately Flashmen manage to destroy Mess before anti-Flash phenomenon can take toll on them.
But there was no time to celebrate, as the anti-Flash phenomenon was about to claim lives of Flashmen and the saviors of Earth was forced to leave planet Earth in haste. Only Yellow Flash succeeded in reuniting her birth parents - but she still couldn't depart properly as she was very sick thanks to Anti-Flash Phenomenon.
Flashman joined up with Battle Fever, Denzimen, Sun Vulcan, Goggle V, Dynamen, Biomen, Changemen, Maskmen, and the Livemen, to stand behind the newest Sentai, the Turborangers, as they faced off against the Hundred Boma Tribes. Counting on You! Turboranger
Years later, the Space Empire Zangyack invaded Earth. This threat was so great that all of the first 34 Super Sentai, including Flashman, were needed to oppose them. The 34 Sentai put up a valiant fight against the ground forces, but when the Zangyack fleet itself attacked them, Akarenger told everyone to combine their powers and save the Earth. This destroyed the Zangyack fleet and resulted in the loss of their powers, which resurfaced as Ranger Keys, that were dispersed throughout the universe.
The keys were collected by AkaRed, and later utilized by the 35th Super Sentai, the Gokaigers, a group of five people from different planets who came to Earth in search of the Greatest Treasure in the Universe. Ep. 1: The Space Pirates Appear The greater power of Flashman was initially taken by Basco ta Jolokia, but was granted to the Gokaigers by Dai after Basco's death. Ep. 49: The Greatest Treasure in the Universe After the Gokaigers finally defeated Zangyack, they returned the Ranger Keys before they left Earth to find the second Greatest Treasure in the Universe. It is presumbed the Flashmen got their powers back as with all the other Super Sentai. Final Ep.: Farewell Space Pirates
Super Hero Taisen
The Flashmen, among the majority of the Super Sentai heroes, were caught up in the "Super Hero Taisen" incident. They were presumably sent to the dimensional riftIcon-crosswiki, under the pretense of being defeated, by Kamen Rider Decade while he was apparently hunting the Sentai teams. When his and Captain Marvelous' ruse was revealed, Flashman appeared with the other Super Sentai through the dimensional wallIcon-crosswiki, they then fought alongside all the other Sentai teams and Kamen RidersIcon-crosswiki against the alliance of Dai-ShockerIcon-crosswiki and Dai-Zangyack. Kamen Rider × Super Sentai: Super Hero Taisen
Super Hero Taisen Z
The Flashmen are seen alongside the Dekarangers, standing behind Space Sheriff Gavan Type G and Kamen Riders Fourze, Meteor, Decade and Super-1 as the three forces of super heroes confront Space Shocker. Kamen Rider × Super Sentai × Space Sheriff: Super Hero Taisen Z