Through the Looking Glass
Through the Looking Glass is a secret mission exclusive to Super Mario 64 DS. The title is a reference to Lewis Carroll's novel Through The Looking-Glass.
Once the player has unlocked Luigi, they must switch to his character and locate the mirror room on the second floor of the Mushroom Castle near the Snowman's Land and Chief Chilly Challenge paintings. Luigi must collect one of the Power Flowers on display to become Vanish Luigi, then enter the mirror. He must then open the door in the back of the mirror, reflecting the one used to enter the mirror room. Luigi finds himself in a completely blank area with almost nothing more than the other side of the door and a Power Star, which he is then able to collect.
If Luigi's vanish ability runs out before he makes it out of the mirror, there are Power Flowers in the mirror that Luigi can use to become Vanish Luigi again.