In the middle of the 19th century, not Europeans but aliens from outer space arrive in Japan. Twenty years after the invasion, Japan is ruled by a puppet shogunate, which makes sure that unscathed patriotic samurai do not raise their voices against the aliens. Wearing katanas and other edged weapons is also banned. Former samurai Gintoki Sakata used to be among the rebels, but now he's done with saving humanity and is mostly lying on the couch when a rumor comes out that his childhood friend has been killed in a sword fight with an unknown samurai.