I almost went through this whole year without realizing it was Sonic's 20th anniversary on gaming consoles! Great game!
Sonic has to go through numerous zones to get to Dr. Robotnik and stop him from his crazed terror of turning animals into his own personal robots. On the way, Sonic also has to collect the Chaos Emeralds if he wants to TOTALLY keep Robotnik from thriving.
Sonic has to reach the bonus part of every level in order to obtain a Chaos Emerald. I actually find these levels more fun to play than the zone levels because of how challenging it is to get the Emerald, and wondering if I am going to get it or fall into the part of the level where it warps me back to my zone.
Overall, a great game. I give it 9.5/10