So is the plot of this that Gintoki travels into the future and runs into Kagura and Shinpachi who had to run the Yorozuya by themselves and therefor became great warriors because of his past guidance to them so that they vowed to become as strong as him so he wouldn't have to protect them forever, and then Takasugi died but Katsura still saw him as a friend, so he had to keep the Kihetai from spreading all over the world and took command over them, and then realizing that with this entire force, he really could overthrow the Bakfu, so without Gintoki to keep him from doing something like that, he managed to succeed, which caused the Amanto to want to destroy Earth, and that's why everything is so desolate and shit, because of the ongoing war?