The Overthere Stair is a long path that leads from the Underwhere up to the Overthere. It is found in Chapter 7 of Super Paper Mario.
There are many enemies there, and lots of clouds that can bounce Mario really high up. There are also many doors that lead to Apples such as the Red Apple, Blue Apple, and Black Apple, all of which heal Mario and Co. to some extent, and seem to have strange effects on other characters. Mario, Luigi, and Bowser find Princess Peach there when they are taking Luvbi to the Overthere, and must awaken her from a deep sleep by finding the right Apple.