The start of this level will allow you to actually grab one of the two potted Piranha Plants, which will instantly devour anything in its path. You don’t really need this for anything, though, since none of the collectibles will require having the Piranha Plant in your possession. Keep moving to the right and you will see the shadow of a star on the far wall, so move towards the screen to find Green Star #1. Continue right to a ?-Block, grabbing the power-up, then navigate your way past the Fuzzies. Keep going to find another potted Piranha Plant, as well as some Fire Piranha Plants and moving platforms, to the level’s Checkpoint Flag.
Enter the door here to gain a new perspective, as you are part of the actual shadow now. As you move right, you will come to a Super Bell floating in the air. When you can drop down below the Mega Piranha Plant, do so, then slash the blocks away so you can reach the Stamp. If you lost the Cat Suit, then rolling will also break the blocks. Exit back into the 3D World and grab a potted Piranha Plant, so it can eat away the Blurkers that hide a Mystery Box.
Inside the Mystery Box, quickly step on the P Switch and run underneath the steps to Green Star #2 . Cross the moving platforms to a door, but don’t go inside. Instead, go to the right of the door to find a secret passage, where Captain Toad is cowering in fear. Move towards the screen and knock down the Bowser Board to receive Green Star #3 from Captain Toad himself. Now exit via the door, then go in the other door to get to the necessary height to grab the top of the Goal Pole.