Gintama is one of a kind anime. It doesn't have a continuity in storyline or a plot. It is mostly episodic with some absolutely great arcs in between. It's the absolute randomness that makes it great. It has a samurai in modern Japan setting, though the samurais are completely useless in this series. Throw in useless samurais together with useless aliens with some really innovative quirky slapstick humor; you have your regular Gintama episode. Though the humor in Gintama takes some time to get used to; but once you are in, its one heck of a journey.
The story revolves around completely useless jack of all trades Gin and his cohorts. It can range from nerve rattling psychological food fight thriller to bloody yakuza fights. The possibilities are endless in Gintama. If you are looking for parodies, quirky humor and breathtaking fights Gintama is a must for you.