The game begins after what happened in the previous game, Sonic the Hedgehog 2. In Sonic 2, Sonic and Tails defeated Dr. Eggman on his giant spaceship called the Death Egg. It then exploded and landed on Angel Island, an island that floats in the air. Knuckles lives on Angel Island. He guards the Master Emerald, which is a big green gem that makes the island float. Dr. Eggman knows that the Master Emerald is very powerful. He wants to steal it, because he could fix the Death Egg with that power. He tricks Knuckles and tells him that Sonic and Tails are thieves and they want to steal the Master Emerald. Soon, Sonic and Tails arrive on Angel Island because they have seen that the Death Egg has fallen there. However, Knuckles surprises them, and takes away the Chaos Emeralds Sonic and Tails have collected.