From the Super Mario Wiki
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Rap Land is built upon rap music, as most of the buildings resemble either stereos or speakers, which constantly play rap songs all day. The citizens of Rap Land are all rappers and live under the rule of King James. Two notable citizens of Rap Land are the Flab Boys.
When King Koopa comes to Rap Land, he takes on the alter-ego of "Rappin' Koopa", ties up King James, and takes over his castle. Rappin' Koopa plays a hypnotic theme on all the speakers throughout Rap Land, luring all of the citizens into a trance, and tells them to give him all their Gold Coins.
Mario, Luigi, Princess Toadstool and Toad meet with the Flab Boys upon arriving in Rap Land and try to figure a way to save King James and Rap Land from Rappin' Koopa. While Princess Toadstool manages to sneak Toad into the castle, she herself is captured by the Koopas and is taken hostage along with King James.
Toad manages to rescue them before they can get stabbed with a giant record needle, and they meet up with Mario, Luigi and the Flab Boys in another part of Rap Land, away from the castle. It is there that King James pointed out Rap Land's power supply: a hydroelectric dam. After Mario unplugs the cord, he succeeds in putting an end to Rappin' Koopa's tyranny.
Mario then trounces the Rappin' Koopa troop in a rapping duel, forcing King Koopa to escape from the city through a Warp Zone. With everything back to normal, Rap Land celebrates the return of King James, while Mario and his friends dance to the Mario Rap.