The thing is, the game is really huge because there are a lot of Pokemon to catch (in this game, if I remember correctly, around 251 plus a few secret ones ;)). All these Pokemon are different and have specific elements (water, fire, grass, electric, etc.), which make them stronger or weaker against each other. The Pokemon you catch are used to battle in the game against other Pokemon and trainers to earn money. Additionally, you need to earn badges in Gyms to progress. Each Gym has an elite trainer specializing in a particular element, and a badge is what you earn when you defeat them. This is the only way to progress significantly in the game because without badges, your Pokemon will eventually stop obeying you.
Besides these exciting aspects, you'll encounter certain criminal organizations, people with varying degrees of integrity, and legendary Pokemon of which there is only one, etc. In any case, there's plenty to do.
Just know that if you buy one of these two, you'll also need the other to collect all the Pokemon. Because some are only in the Red version and others only in the Blue version. (It might be fun if you play with a friend, you can trade the missing ones).
But well, that pretty much sums it up. Maybe my story is a bit long-winded. But there's so much to tell, and it's challenging to explain this game briefly and succinctly.
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