The first 3 sets of 4 each volumes take place on Ash's journey in Kanto and the last 2 issues of the 3rd volume is the closure of the first season of the cartoon. However, volume 4 contains Ash's journey, also in its own way, through the orange islands, aka season 2.

All of the story is unique and has a fresh look at the Pokemon world. Ash however does not necessarily catch the same pokemon and also does not use the same tools nor training methods as used in the cartoon. Although he still uses a Pokedex, it looks much more futuristic as does a lot of the world in the storyline.

It is a great read, if you can find a copy of the comic that is.