the player controls two sonic the hedgehog characters: classic sonic and modern sonic. the player has to collect all of the chaos emeralds and stop an unknown villain from creating a time hole that would change history in a bad way. the game features levels that come from a past game in the "sonic the hedgehog" series. each level has three stages and two bosses. the stages are classic, dreamcast, and modern. classic sonic stages are 2d stages with gameplay in the style of the 2d "sonic games". modern sonic stages are 2d/3d stages with gameplay in the style of 3d "sonic" games. each level has an act for both sonic characters, as well as 10 challenges such as racing an opponent to the goal.