So after Knuckles gets the master emerald back, Eggman attempts to attack angel island resulting in the island catching on fire! Knuckles knows he needs the flame emeralds to fix this. So he goes on a quest to find them

So Knuckles goes down to the Hidden Palace to see if he can find some emeralds there. And Sonic is there too! When Knuckles learns that he has the purple emerald, they fight it out. Last one standing gets the emerald.

It was a nice day on Angel Island until Metal Sonic snatched the Master Emerald when he wasn’t looking! Knuckles try’s to get the Master Emerald back, but fails. Knuckles knows he needs the power of the Chaos Emeralds to defeat METAL SONIC and get back the Master Emerald, so he goes on a quest to find the emeralds and some coins along the way