Seeking revenge on Wario after he stole their treasure in Wario Land: Super Mario Land 3, special agents from the Black Sugar Gang of Kitchen Island sneak into Wario's newly built castle, steal back the treasure, and leave the castle and the surrounding area a mess. As Wario, you must clean the place up and take back what you now call your own.
In a change from the original Wario Land, Wario is now immortal, to a certain point. Instead of dying when hit by an enemy, Wario loses the coins he collected along his travels in each level. And he can also turn into other Warios. For example, if he is hit by a bee, Wario puffs up into a giant ball of air, and floats up to the top of the level. Only when hit by a sharp object will he return to normal. There are other types, like Fire Wario and Zombie Wario, and each can have its advantages, like getting to other areas otherwise unreachable, or they can halt your progress.