One evening, Mario had a strange dream. 
He dreamt of a long, long staircase leading up to a door. As soon as the door opened, he was confronted with a world he had never seen before spreading out as far as his eyes could see. When he strained his ears to listen, he heard a faint voice saying 
"Welcome to 'Subcon', the land of dreams. We have been cursed by Wart and we are completely under his evil spell. We have been awaiting your arrival. Please defeat Wart and return Subcon to its natural state. The curse Wart has put on you in the real world will not have any effect upon you here. Remember, Wart hates vegetables. Please help us!"
At the same time this was heard, a bolt of lightning flashed before Mario's eyes. Stunned, Mario lost his footing and tumbled upside down. He awoke with a start to find himself sitting up in his bed. To clear his head, Mario talked to Luigi, Toad and Princess about the strange dream he had. 
They decide to go to a nearby mountain for a picnic. 
After arriving at the picnic area and looking at the scenery, they see a small cave nearby. When they enter this cave, to their great surprise, there's a stairway leading up, up and up. It is exactly like the one Mario saw in his dream. They all walk together up the stairs and at the top, find a door just like the one in Mario's dream. When Mario and his friends, in fear, open the door, to their surprise, the world that he saw in his dream spreads out before them!