Gin Tama ( "Silver Soul"), also known as Gintama, is a Japanese manga written and illustrated by Hideaki Sorachi and serialized, beginning on December 8, 2003, in Shueisha's Weekly Shōnen Jump. Set in an Edo which has been conquered by aliens named Amanto, the plot follows life from the point of view of samurai Gintoki Sakata, who works as a freelancer alongside his friends Shinpachi Shimura and Kagura in order to pay the monthly rent. Sorachi added the science fiction setting to develop characters to his liking after his editor suggested doing a historical series. 
PLOT
Twenty years ago Edo-period Japan suffered a great cultural  shock—the aliens called Amanto invaded the earth, overpowered the  nation's government, and outlawed the carrying of swords by the once  proud samurai. Moreover, the aliens took the people's jobs and are  currently running Japan as a sweatshop using the natives as laborers. In  this strange new era are Gintoki Sakata, an eccentric samurai whose  personal trademarks are his naturally permed silver hair, his love of  anything sweet, and his addiction to Shonen Jump; Shinpachi  Shimura, the teenaged heir of the Kakido-Ryu style; and Kagura, an  incredibly strong alien girl from the powerful Yato clan. The trio are  self-employed in an odd jobs shop called Yoruzuya Gin-chan, which offers  to do almost anything for a price, from finding lost kittens to saving  the world. Unfortunately, the jobs rarely go as planned, or pay enough  to cover the pain and suffering the employees encounter—never mind their  rent.
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