The story of SU in its entirety.
Once, all of Mobius was peaceful and beautiful. It was ruled by the kind hearted, loving Queen Aleena. All that peace came to an end, however, when Dr. Robotnik, a trusted army leader, used his technology to throw Aleena off the throne and took over as the Supreme Tyrant of Mobius. From that day on all of Mobius was in tyranny. Forced into hiding, Queen Aleena gave birth to three beautiful children: Sonic, Sonya, and Manic.
It was soon after her children were born that the Oracle of Delphius revealed a prophecy of the anchients to the Queen. The prophecy stated that someday she and her children would form the Council of Four and overthrow Robotnik and restore peace to their planet. There was a catch though: in order for the prophecy to be fulfilled, Queen Aleena had to give up her children.
Heart heavy but determined to end the tyranny once and for all Aleena did just as the Oracle told her to. She dropped Sonya on the doorstep of a wealthy lady, Sonic was put on the door step of a mountaineer couple's home, and Manic was dropped off at an old lady's house (but was then stolen off the doorstep as soon as Aleena fled the scene). The duty done Aleena continued to run from Robotnik and remain hidden but she always remained close to her children so she could watch them grow.
After fifteen long years the triplets finally reunited and formed a band they called the Sonic Underground. Since then they have gone on a long quest to restore freedom and peace to their planet and to find and reunite with their mother in the hopes of getting rid of Robotnik once and for all.