After Dr. Robotnik overthrows Queen Aleena, her children Sonic, Manic and Sonia are seperated with her. When three children grow up, the Oracle of Delphius gives them powerful weapons to reclaim their planet: the medallions that can change into musical instruments. With the medallions, the three form an underground rock band, "Sonic Underground" and find the way to reunite with Queen Aleena and defeat Dr. Robotnik.