Nothing could possible describe this game as well as the instruction manual:
One day the kingdom of the peaceful mushroom people was invaded by the Koopa, a tribe of turtles famous for their black magic. The quiet, peace-loving Mushroom People were turned into mere stones, bricks and even field horse-hair plants, and the Mushroom Kingdom fell into ruin.

The only one who can undo the magic spell on the Mushroom People and return them to their normal selves is the Princess Toadstool, the daughter of the Mushroom King. Unfortunately, she is presently in the hands of the great Koopa turtle king.

Mario, the hero of this story (maybe) hears about the Mushroom People's plight and sets out on a quest to free the Mushroom Princess from the evil Koopa and restore the fallen kingdom of the Mushroom People.

You are Mario! It's up to you to save the Mushroom People from the black power of the Koopa.

This game seriously has as many subtle drug references as Alice in Wonderland. The whole thing about the mushrooms is downright freaky, and the weed that takes you up in the clouds? Nintendo, I'm ashamed at you.

Here are some wonderful Mario Links:
Super Mario Bros. Super Site
Super Mario Bros. Villians
Communist Mario