Super Paper Mario/Chapter 1: Lineland

When you go through the red door, you'll find a land with algebra equations in the sky called Lineland.

The Adventure Unfolds
Enemies: Get it? Unfolds? Paper? Ha ha. Yeah. Anyway, just move far to the right side of this screen, jumping on the enemies to kill them. In the house there, use Tippi to reveal a hidden door. You'll meet Bestovious, who will teach you how to flip to 3D (Press the A button as Mario). When you do, a meter will appear at the top of the screen. If it runs out, you'll lose 1 HP, so be careful!
 * Goomba - 1 HP, 1 Attack, 0 Defense
 * Squiglet - 2 HP, 1 Attack, 0 Defense
 * Sproing-Oing - 4 HP, 1 Attack, 0 Defense

Go back outside, then flip near the door surrounded by blocks in order to get into it. In the next area, continue going right and flip near the green pipe to pass it. When you get to a giant gap, flip again and walk on the path to the left.

When you get to a large wall that seems impossible to pass, flip and keep going. In the next area (yup, another one), hit the ? block to get a Mega Star that makes you...MEGA MARIO!! You can kill any enemies and break bricks by touching them. When your power runs out, you'll find a Star Block, which you can hit to end this chapter.

Afoot in the Foothills
Enemies: You'll now have to cross Mount Lineland to get to Yold Town, which leads to the Pure Heart. Jump on the Koopa Troopas at the beginning to knock them into their shells, then kick them to get points and crash into other foes. Climb the ledges until you get to large, moving rectangle. By standing in the red square near them, these rectangles will scoop you up and take you to the other side of the area. Run right to find a switch; activate it in 3D to raise ledges leading to more rectangles. Ride them and go through the door.
 * Koopa Troopa - 1 HP, 1 Attack, 1 Defense
 * Paratroopa - 1 HP, 1 Attack, 1 Defense

Keep going right until you seem to reach a dead end, then flip into 3D to find some blocks you can use as steps. Do the same for the next set. Defeat the Koopas here, and proceed towards the incline. Some Spiky Tromps are rolling down it. They are nearly impossible to jump over, so use 3D and go through the door. You'll find a large gap that you cannot jump across. Go into the nearby pipe and go into the house. Go up the stairs and flip into 3D to find Red, who will put down the bridge for you. Continue across the bridge to Yold Town.

If you want to, you can talk to the people here or visit another one of Howzit's shops. When you're ready to continue, go to the mayor's house on the far right to hear that if you are rally the hero, you must have a Pixl shaped like a hand. To find it, enter the house to the left of the mayor's (whose name is Watchitt), flip, and enter the pipe behind the books. To avoid the Thwomps, flip again and go through the door. You'll come to a seemingly empty room, but wait! Use Tippi to find a hidden door. Here, you'll find the Pixl Watchitt was talking about, Thoreau, but you'll be become locked in the room where you found him. To get out, grab the nearby block with Thoreau by pressing the 1 button, then jump and press it again near the switch. Return to Watchitt, who will tell the bridge...guy to put down the bridge. Go to his house by using the pipe near Watchitt's. Talk to him (his name's Green, by the way) and you'll be able to cross the the Star Block.