From the Super Mario Wiki
The level begins with Mario in front of the cannon on the world's overworld. Entering the cannon initiates a countdown, where he is shot into the main portion of the level, dashing without stopping until the end. A Crowber will first appear, followed by a Boomerang Bro. A Red Ring can then be found. More Crowbers and Boomerang Bros. follow. After that, the level ends, unlocking World Flower-1.