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Game is set in a region where only established gym leaders are able to challenge the champion. You start the game by defeating a gym leader who is then forced to hand over leadership of their gym to you.
You spend the rest of the game fighting the other gyms to prove yourself as the strongest leader and earn recognition from the champion. As you travel the region trainers come to your gym demanding to challenge you and take over your gym, so you have to return periodically to retain your title.
You hire other trainers throughout the game to be your underlings, and the stronger they are the more trainers they prevent from getting to challenge you. You can customize your gym to represent a pokemon type (fire, water, grass, etc.) or a more general theme with varied pokemon types (delinquent, clown, wealthy, etc.).