Sonic Underground: Secrets of the Chaos Emerald (2008)

"They made a vow, their mother will be found!"

Secrets of the Chaos Emerald focuses on Sonic, Manic and Sonia as they help Knuckles, Floating Island's stern guardian of the Chaos Emerald, protect the Chaos Emerald from Dr. Robotnik and his henchmen, Sleet and Dingo. If the Chaos Emerald gets into the wrong hands it can be used as a weapon against Sonic and Mobius, but at the same time the Chaos Emerald can be used to save Mobius - there has never been so much pressure on Sonic and his siblings.