After the events of Sonic CD, Dr.Ivo Robotnik discovered the emerald shards, these were shards that had so much power that they could destroy an entire island.

After Robotnik took them to his new lab called: Tech Egg, he builted a machine which uses the power of the master emerald and the emerald shards, he aimed for angel island and fired at it, meanwhile… Tails and Knuckles were setting up a party for sonic in South Island.

Sonic himself was still running through Green Hill Zone, until there was a huge blast and before they knew, the three of them saw that Westside Island and Angel Island had Fused together, Sonic called Tails and Knuckles and together they set course to the Fused Island and stop Dr.Robotnik for once and for all!