In a cruel twist of fate Mario’s ghostbusting brother, Luigi, is stuck in a haunted house in Super Mario Galaxy. I guess Luigi must have left his Poltergust 3000 at home. You can spot Luigi outside the Boo filled mansion, but he is behind bars. The only way to rescue Luigi and add an extra star to your collection is to walk inside.
Goombas wearing pumpkins heads patrol the house. Their makeshift helmet protects them from a basic bounce on the noggin. However, if you knock them with a spin attack the shell breaks rendering the goomba defenseless. When their pumpkin helm is on these goombas also have a special ability, they drop stationary blue flames.
Inside the house is populated with Boos (aka Teresas in Japan) and you can't stomp on these guys. If Mario can get a Boo to waltz into sunlight it will vanish and leave a coin behind. Later in the stage Mario becomes a Boo by eating a ghastly looking mushroom. When Mario dons the Boo suit he can pass through fences by waving the remote. However, you can’t simply float into Luigi’s cell. You need to pass through a series of fences while avoiding beams of light to rescue Luigi.
This level is one of the rare places you get to use the Boo suit. While Super Mario Galaxy has a handful of power-ups they are stage specific. That means most of the game is spent as plain old Super Mario. Even in the odd level where you find a fire flower, the time you spend as chucking fireballs by shaking the remote is limited. After a few seconds the Fire Flower power up fizzles out and you’re back to normal. The same goes with Ice Mario, you're limited to a few seconds of lake freezing power.
I guess if Nintendo gave you free reign to be Boo suit Mario it would probably ruin the game. Unlike Bee Mario, Boo suit Mario can fly for an indefinite amount of time by tapping the A button. This ability alone would make Super Mario Galaxy way too easy. But for this level it's a good thing Boo suit Mario floats. Luigi is trapped slightly higher than Boo suit Mario passively hovers. The only way to enter the cage and save him is to float up. Luigi won't recognize Mario if he is wearing the Boo suit. You have to transform back into Mario by touching a moving light beam. Once Mario is in his familiar clothes Luigi gives Mario the power star he found.