Old Jouishishi ( 旧攘夷志士 ) refers to the samurai who participated in the Joui War and fought against the Amanto, extraterrestrial beings from space.
Twenty years before the present, the Bakufu was resistant to the arrival of the Amanto, but they were confronted with the Amanto's superior technology and weapons. When the 12th Shogun collapsed due to anxiety, Tokugawa Sada Sada took the position of Shogun and welcomed the Amanto to their country.
Some samurai within the country were against this and lost faith in the Bakufu, calling them traitors. This caused a long civil war called the Joui Wars which lasted for about 10 years. Those who participated in the Joui War were known as the Old Joui Patriots.
Saigou Tokumori was a famous Jouishishi with the nickname "White Loincloth Saigou". He fought against Amanto with nothing besides from his monstrous strength and was usually seen wearing only his loincloth. Saigou would later retreat from the battlefield and return to his family, eventually opening an Okama bar and welcoming Old Joui Patriots with no place to call home.
Terada Tatsugorou and Doromizu Jirochou, both of whom loved the Kabuki district, joined the war in order to protect it, but Tatsugorou was killed in action while Jirochou retreated. Jirochou knew that they were fighting a losing war, so he returned to Kabuki District and ended up abandoning his family. He controlled his Yakuza group with an iron fist to stop the Amanto such as Kujaku Hime Kada from invading Kabuki District.
While the Bakufu were fighting a war, the Tendoshu intervened and asked the government to conduct the Kansei Purge. They hunted down potential and actual Joui rebels and displayed the decapitated heads to the public to deter resistance. A sudden decline to Joui activity occurred. But because of the arrest of Yoshida Shouyou, his students rose up and sparked the last revolt, lead by three of his students: Sakata Gintoki, Katsura Kotarou and Takasugi Shinsuke.
These three as well as the later addition, Sakamoto Tatsuma, would become known as the Last Generation as well as being dubbed the Four Heavenly Kings. Gintoki earned the nickname Shiroyasha (White Demon), feared by comrades and enemies alike for his demon-like fighting style. Takasugi created and led his own force called the Kiheitai (Demon Army) and became known for his successful battle strategies. Katsura earned the mocking nickname, Runaway Kotarou, for his tactical retreats that minimize ally losses and create better battle plans; he also earned the nickname The Noble Youth of Madness for his poise in battles. And Sakamoto, with the nickname The Dragon of Katsurahama, was able to raise money, weapons and men for the rebel cause.
Unfortunately during a battle, Sakamoto was heavily injured on his sword arm by the Amanto mercenary Pluto Batou. He was taken off the front lines to recover and it was during this that the rebel forces were attacked by the Tenshouin Naraku and massacred, except for Gintoki, Takasugi and Katsura, who were captured. A Tendoushuu member gave Gintoki the sadistic choice to either execute Shouyou (who was forced to see the corpses of his students) or Takasugi and Katsura. Choosing to keep his promise to Shouyou to protect his comrades, Gintoki killed his teacher, sparking a rift between the trio, especially between Gintoki and Takasugi, as the former had broken his promise to the latter to protect Shouyou. Takasugi would later lose his left eye thanks to one of the elite Naraku members, Oboro, who decided to let the trio live and deal with the despair of their failure. After burying their teacher's head and their dead comrades, Takasugi, Gintoki and Katsura separately left the war.
Both Takasugi and Katsura would continue their Joui activities underground. Takasugi reformed the Kiheitai for the purpose of destroying Japan, even resorting to making deals with his former Amanto enemies to complete it. Katsura would form and lead his own extremist Joui faction, later becoming more moderate due to meeting Gintoki again in the series proper.
Sakamoto recovered from his injuries but will never be able to hold a sword again. He learns about the disastrous battle his friends were involved in and tries to find them, but only managed to find Gintoki. Sakamoto tells Gintoki his decision to leave the war for space, to try to seek a way that both humans and Amanto can benefit together instead of meaningless death for everyone. After leaving, he was captured by a space pirate fleet named Chidori known for human trafficking. Thanks to one of its members, Mutsu, Sakamoto was able to overthrow the pirates and free its slaves, establishing the space merchant fleet Kaientai with them and the captured ships.
At the end of the war, a last purge was conducted to kill the patriots, collaborators and their families who were mainly involved in one of the last battles the patriots won that caused the Bakufu general of that battle and his family, the Hitotsubashi, to lose standing. A wandering Gintoki took the place of one of people meant to be captured and executed, the future Ikeda Asaemon, to save her. After being tortured and jailed, the sympathetic executioner, the 16th Ikeda Yaemon, decides to illegally free him. The weakened Gintoki wanders to a graveyard where he meets Tatsugorou's wife, Otose, who takes him in. He will later form the Old Yorozuya then later on, the new Yorozuya.
Hiraga Saburou was also a part of the original Kiheitai. Takasugi Shinsuke has noted that Saburou was not skilled in swordsmanship but was an expert in machinery. After the war, Hiraga Saburou wanted to return to his father's side, but he was caught in the Kansei Purge and killed.
Kurokono Tasuke was nicknamed the Fifth Man for his talent in helping the Four Heavenly Kings from behind the scenes. He will later leave the war as he felt that he will always be inferior to them and easily forgotten due to his lack of presence. In the present, he sent an invitation to the Kings for a reunion and will later save Gintoki, Katsura, and Sakamoto from the Kiheitai, who decided use the party to kill them. It is implied that they intercepted their boss' invitation before Takasugi actually knew about it.