After Mallow gets his Frog Coin back from Croco, you return to the Mushroom Kingdom to find it in the midst of an invasion.

Shyster Invasion

When you return to the Mushroom Kingdom, you find Shysters chasing the townspeople. You can optionally battle each one, and you will get rewards from the people you rescue. Be sure to battle the Shyster that is chasing the person in the south part of town. You can return the Wallet to them afterward to get a Flower Tab. You can get another Flower Tab by defeating the enemies in the western house.

Rest in the inn if needed, then go into the castle. In the southeastern room of the castle, go northeast into the guest room and talk to the person there to get a Wake Up Pin. Then go southwest into the vault and hit the three treasure chests. This is the last chance to get these items, so don't miss them.

Then when you're ready, go northeast to exit the vault, go northwest to enter the main hall, then go northeast into the Throne Room. After a cutscene, go northeast to fight Mack.