That's the hole point.
It doesn't need one.
To appreciate or even start to understand Cowboy Bebop you have to watch the whole thing. but in Samurai Champloo you can just enjoy single episode, since each one tells a complete story.
This way you don't have to watch through 100 boring episodes for the storyline to evolve, like in naruto or inuyasha, or Cowboy bebop for that matter.