Mario & Luigi: the underwater kingdom

Mario & Luigi: the underwater Kingdom is an RPG game in the Mario Series. More specifically the Mario & Luigi sub-series.

Plot:
Mario and Luigi are sent as diplomats to a new underwater kingdom discovered by Toadsworth. Once they are there, they are locked up by the king after being framed for stealing his crown. In prison, they meet Wario and Starlow, who had both been framed as well for other crimes. They expect it to be Bowser, so they escape to the mushroom kingdom only to find Bowser had done no wrong actions...yet. So they go back and forth in their quest to find the truth. In the end, it turns out to be Fawful and Popple (who was manipulated by Fawful). Finally, Mario, Luigi, Starlow, and Wario battle Fawful and Popple. Once they defeat them, they are both arrested. After the credits, it is shown Fawful escaped the prison while Popple hadn't.