In 1987, Nintendo released a Japanese game called Yume Kojo: Doki Doki Panic for the Famicom. The plot was that two children were reading a book when they get kidnapped by a evil frog named Wart and 4 heroes have to save them. A few years later, Nintendo decided to re-release this game in the West and make changes to the game. The main change was that they replaced the 4 playable characters with the Mario characters, which were Mario, Luigi, Peach and Toad. This game became popular because there were Mario characters in the game.
The game retains the enemies, bosses, music and worlds from the original Japanese game as well as making a few changes here and there. Super Mario Bros 2's plot takes place when Mario has a dream that he walks up a stairway to a door and he enters a world called Subcon, where he has to save the world from an evil frog named Wart. He told Luigi, Peach and Toad about this and they also had the same dream as well. They saw a cave, entered it, went up the stairs and saw the same door that led them to Subcon, their dream world! Like their dreams, the world was in danger and they had to stop Wart.
Although this game is a departure from the normal Super Mario Bros games, this game stands as one of the most unique Mario games in the series. It retains the platforming of Super Mario Bros while making a whole new game with it's new formula of picking up objects and throwing it at enemies. Super Mario Bros 2 is an awesome game in the series, despite being based on Yume Kojo: Doki Doki Panic, a game which inspired Super Mario Bros 2. I think that Nintendo should re-release that game as well as Super Mario Bros 2 so people know more about the game which started Mario Bros 2 in the first place.