Pretty much just Red and Blue, but longer and with even more fucking Pokemon you have to catch.
These third generation games find Pokemon players in a whole These games, which at first seem very different from previous games, are actually exactly the same. The player recieves a starter Pokemon, battles their rival at multiple points throughout the game, collects 8 badges, stops a villainous team's plans for world domination, and eventually battle the elite 4 and become the champion. Just like every other Pokemon game out there.
You start out after your family moves to Littleroot Town, so named because it was the only thing the game designers could think of that was gayer than New Bark Town. You save Proffesor Birch from a wild...dog thing, and e gives you a Pokemon in return. But just as always, it comes with the hidden terms that you have to catch every Pokemon out there. (As a side note...this is impossible)
You set out and battle through eight gym leaders (one of whom is your dad...WTF?) and eventually meet a villainous team. In Ruby, your enemy team is Team Magma, and your ally team is Team Aqua. In Sapphire, this is reversed. And you lucky bastards playing Emerald get to have them both as your enemies. Team Magma's evil plan is to awaken Groudon (that Charizard fish up there) to create more land mass. Team Aqua wants to awaken Kyogre and create more oceans. Neither of these groups really has a reason, they just want to fuck around with Earth a little bit. Anyway, you have to go and either defeat or capture the raging Pokemon. Then, in one of the most dramatic moments in video game history, the evil team just kind of goes away.
So then you go battle the Elite 4. Contrary to popular belief, these bastards are fucking easy to crush. You then go on and face the champion, who you actually met earlier on a worthless side quest. You challenge him, and become the champion. Your reward? You get to go to a tower on a faraway island and battle more NPCs. Jolly good fun.
The three starters of this region are pretty much shit. You get Treeko, a grass type gecko; Torchic, an orange bird with his head on fire, or Mudkip, a fish...with legs.
This game also rips off a few classic Gen one pokemon...
etc, etc, etc