Courtesy of Nintendo Power:

Become the greatest Pokémon trainer in the world! Start your Pokémon Game Boy adventure by choosing a tame Pokémon at the home of the great Pokémon master Professor Oak. Then start your journey.

Your mission: Collect all 150 Pokémon, just like Ash, using your own Pokémon to capture others. Along the way, several skilled trainers, such as Gary, and many minor trainers will challenge you to Pokémon duels. You can't back down, so be ready for anything!

Remember, Pokémon are not ordinary creatures. To collect all 150, you'll need to train captured Pokémon so they evolve. For instance, a captured Caterpie will evolve into a Metapod, which will then turn into a Butterfree. Each transformation adds more power to the Pokémon-giving you more power to defeat and capture other Pokémon.

Also keep in mind that some Pokémon are rare and won't be found in your game. Newer Pokemon from Pokémon Gold and Silver do not appear in this game, either.

The Red & Blue versions of the game each carry their own variety of Pokémon to defeat, collect, and train. To get all 150, trade Pokémon with your friends using the Game Link Cable, which allows the transfer of Pokémon between Game Paks.