Having cleared the first three Zones, the special Challenge Gates have been unlocked. In order to progress, Sonic must find three special Keys which can be obtained by completing just one Challenge in each of the three Zones. Of course, Sonic isn't going to stop at just one Challenge, as completing them all will restore even more of the world and unlock special rewards. There are ten Challenges per Zone, split evenly between Classic and Modern Sonic.
Classic Sonic gets to attempt the Challenges first, and along the way meets up with his greatest rival, Metal Sonic, in a battle to claim one of the seven Chaos Emeralds...
