Mario begins this level on the Unknown Planet, which contains a Launch Star in a cage. Mario will have to lure a Bullet Bill over to the cage to free the Launch Star and reach Megaleg's Planet. Bowser Jr. will appear and summon Megaleg. Mario must lure two Bullet Bills in quick succession; one into the outer barrier, and then another into the Grand Star cage to destroy it before the outer barrier repairs itself. After defeating Megaleg, Mario earns a Grand Star.
This is a very small planet. Mario simply has to get a Bullet Bill to break open a piece of machinery to reveal a Launch Star, and then use that Launch Star to reach Megaleg's Planet. The planet's center is a Black Hole, and there are holes scattered around the planet.
It is fairly small and orange, with many technology symbols all over it. It has small rectangular areas on it that are transparent and seem to have computer components inside them. It where Megaleg is battled.