Subcon (sometimes called Sub-Con; also known as Mushroom Kingdom II in Super Smash Bros. Melee) is a peaceful dream world of the Subcons. Initially believed to be Mario's dream, it is said to be a vast dream world that is part of a larger subconsciousness of the Mushroom World's inhabitants. The name "Subcon" itself is short for "subconscious".
In Yume Kōjō: Doki Doki Panic, Subcon (then called "Muu") was in a storybook that the kids Piki and Poki get pulled in by Wart ("Mamu") and get locked in a cage. In fact, the reason Chapter 7 is only two pages is due to the fact that the original ending was accidentally destroyed. This detail is lost in the Mario version of the game's events.
 Super Mario series
Subcon is the main setting of the game Super Mario Bros. 2, where the world was first available. Subcon was once a peaceful place, until it was taken over by the tyrannical frog king Wart and his army. He then enslaved the inhabitants of Subcon to be his soldiers. Wart captured the Subcon fairies, the protectors of the realm and imprisoned them so that they would not interfere with his evil plans. While he resides in the Castle of Subcon, Wart created monsters with the Nightmare Machine, and sent his army all around the dream world. While he fully succeed to take over the world, Wart became the king of Subcon himself.
From their confinement, the Subcons sent a message to the hero of the nearby Mushroom Kingdom, Mario, to come and deliver them from the dictator. At first Mario thought the message was just a dream, but the next day, while on a picnic with Luigi, Princess Toadstool, and Toad, he discovered a cave that led him to the very world he saw in his dream.
Mario and his friends instantly set out to free the fairies and defeat Wart to free Subcon from his reign. They battled their way through Wart's army, the 8 Bits, and his generals: Birdo, Mouser, Clawgrip, Fryguy, and Tryclyde. In the end they faced the evil king himself, and were eventually able to defeat him and free the captive fairies. They then awakened to find themselves back in the Mushroom Kingdom.
Subcon is again the main setting of the pseudo-sequel of Super Mario Bros. 2 for Satellaview, BS Super Mario USA. This game reveals that Subcon is a dream world, rather than an actual dream like the ending of Super Mario Bros. 2 implied, and explains that after his defeat, Wart and the 8 Bits went into hiding in another dream realm for a short while. However, they soon return, and the king of Subcon is forced to use Star power to call Mario and the others back to Subcon to stop Wart once again. Mario, Luigi, Toad and Princess Toadstool were able to defeat Wart and his crew once more, thus returning peace to Subcon and its inhabitants.
 Super Mario Advance
In the Super Mario Advance remake of Super Mario Bros. 2, there were now Yoshi Eggs scattered all around Subcon, which Mario and friends has to recuperate. Besides that, the game's depiction of Subcon is the same as in the original.
Subcon made its first Super Smash Bros. series appearance in Super Smash Bros. Melee, as a stage called Mushroom Kingdom II, which is based on World 1-1 and 1-2 of Super Mario Bros. 2. Mushroom Kingdom II is the second Mushroom Kingdom stage in Super Smash Bros. Melee. It can be unlocked by collecting the Birdo or the Pidgit trophy. Unlike the first Mushroom Kingdom stage, Subcon has 8-bit graphics converted into 3D and 16-bit graphics that remain 2D. This stage doesn't take place in the Mushroom Kingdom, despite the name.
 Physical Features
Plants stick out throughout Subcon, which, when pulled, will give the character an item (or occasionally, a Bob-omb). A lot of the region of Subcon is high in the sky; the "land" being clouds. The dream world has many moons at night as first seen in Super Mario All-Stars. Subcon also has many recurring landscapes, such as deserts, snowy lands, islands, mountains, grass plains and others. The only castle is Wart's castle, and no towns or cities is seen throughout the game. However, according to BS Super Mario USA, Subcon has a kingdom and a king. It is not known how much of Subcon is considered a true kingdom; if not the entirety of the dream world, seventh world in the clouds of Subcon above the grass or the desert region is probably the whole region of the Subcon Kingdom itself. Wart's castle from World 7-2 is the king's castle, which was in the sky.
Subcon also uses Coins, though their design is different with a "I" mark on it instead of "1" from Mushroom World. Coins in this world are rather rare. Instead of being found floating in the air on Mushroom World, for unknown reasons, Coins could only be retrieved by using a Magic Potion to create a door near unpicked vegetables and lead to Subspace, where the Coins are found, but could not be found if the vegetables are already picked outside Subspace. Coins are mostly known to be used to earn prizes (1-ups) in a minigame after finishing a level.
Subcon's population consists many species, some are unique to the setting, while others were later portrayed as inhabiting the Mushroom World as well. Most prevalent are Subcons, Shy Guys, Bob-ombs, Ninji, Albatrosses, and Pokeys. The King of Subcon is human, there are very few humans living in Subcon as well. Species from the Real World, like the Whales and Snowmen, reside on Subcon's icy land.
 Species of Subcon
 Protectors of Subcon
 Enemies of Subcon
 Names in Other Languages