Source taken from Wikipedia.org
Gin Tama literally "Silver Soul" is a Japanese manga written and illustrated by Hideaki Sorachi and serialized, beginning in 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.
The story of Gin Tama takes place in Edo (known today as Tokyo because of its name change in 1868), Japan, which was conquered by aliens called Amanto in the late Edo period. The samurai of Japan fought the aliens, but after their defeat the Amanto placed a ban on carrying swords in public.
