Super Mario Bros. is a platform video game developed by Nintendo in late 1985 and published for the Nintendo Entertainment System as a sequel to the 1983 game Mario Bros.. In Super Mario Bros., the titular character, Mario, saves Princess Toadstool of the Mushroom Kingdom from King Koopa (later renamed Bowser), king of the Koopas. Mario's younger brother, Luigi, is playable by the second player in the game's multiplayer mode, and assumes the same plot role as Mario. To save Princess Toadstool, Mario conquers the eight worlds of the Mushroom Kingdom by going to the castle in each to defeat a minion of King Koopa. To reach each castle, Mario battles through three "sub-worlds" by defeating or avoiding King Koopa's henchmen. If Mario successfully fights his way through the castle and defeats the minion, a Mushroom Retainer is freed. Inside the eighth castle, Mario has a final fight with King Koopa and free Princess Toadstool.