There is a plot, you fight an entire organization (their motives vary depending on the version), strive to beat your rival and the Elite 4...that's pretty much it, the rest of your game is your own adventure that consists of walking through the environments, talking to guys, collecting items, making berries, beauty contests, battling...a lot of fun things to do.
And the battles themselves are pretty deep actually, there are lots of strategies and damage calculations to consider if you want to be good.
I hated Misty the first time I played Pokemon Blue.
Her ass kept using harden then when I finally got Starmie's HP down she wants to switch it out or use a potion. Bitch please.