the best sonic game ever!!!!!!
you get to play as both classic and modern sonic. it all started when a creature known as the time eater is tearing time and space and sucked sonic and all his friends in a white limbo. both classic and modern sonic must get through every level to restore time and space and save sonic's friends. in all there are 9 levels 3 rivalries and 4 bosses.
it all split up into 3 eras.
the first being the classic era with the levels green hill, chemical plant, and sky sanctuary with metal sonic as the rival in stardust speedway and SPOILER ALERT death egg robot as the boss.
the second being the dreamcast era with the levels speed highway, city escape, and seaside hill with shadow as the rival in final rush and perfect chaos as the boss
the final era is the modern era with the levels crisis city (its better than the one from sonic 06), rooftop run, and planet wisp with silver as the rival in crisis city and egg dragoon as the boss
you can collect chaos emeralds from rivalries and bosses. after beating the levels and rivalry you have to collect three time keys to unlock the boss and the way to do that is to complete one challenge from every level. once you get the keys then you can go to the boss. once you complete the boss you get a chaos emerald
after you collect all 7 chaos emeralds you finally get to beat the boss time eater.