Going to Bowser's Castle Spoiler:as Mario and Luigi and going down a blue pipe in the concession stand area will lead you to an underground room. Entering the correct code in the following room will Spoiler:open a vault, and entering the vault one finds a large icy room. Hitting the button with a hammer will cause some of the ice to melt, revealing three Spoiler: Shroob enemies from the previous installment in the series, Mario and Luigi: Partners in Time.

After the battle, Starlow finds out that these creatures once invaded the Mushroom Kingdom, and that Mario and Luigi Spoiler: defeated them as babies. Starlow also finds out that Spoiler: Luigi defeated the Shroobs by crying in the past, another nod to Partners in Time . This almost assures that the Mario and Luigi in Bowser's Inside Story are Spoiler: the babies in the previous game.