One of the rare Sonic games that don't suck.
Story - For one month out of every year, a tiny planet appears in the skies above Never Lake. This mysterious "Little Planet" holds seven gems called Time Stones that can control the passage of time, bending the barriers between past and future. Intrigued by the unusual power of these stones, Dr. Robotnik descends upon Little Planet and hatches a new scheme to take it over.
Fortunately, Sonic the Hedgehog has caught wind of Robotnik's plot and resolves to beat him to the Time Stones at all costs. However, to complicate matters Dr. Robotnik has kidnapped the animals and the young lady hedgehog Amy Rose and uses them to bait Sonic and keep him off his trail. To that end, he deploys his most sophisticated and diabolical machine to date: a robot duplicate of the hedgehog hero called Metal Sonic, designed to match Sonic's every move and even match his own trademark speed.
Gameplay - Sonic CD's gameplay remains similar to that of Sonic the Hedgehog but with the addition of the Spin Dash and the Super Peel Out, which lets him zoom into a quick speed from a standing point.
Past and Future - You can travel to four different versions of each zone, each a different time period of the same location: Present, Past, Good Future and Bad Future. This is accomplished by speed posts scattered around the level, bearing the labels "Past", and "Future". After running through one of these posts, you have to run at top speed for a few seconds without stopping, to travel into the respective time period. Because these teleports are relative, there are no "Past" signs in the Past, and no "Future" signs in the Future; that is, warping to the past in the future returns the player to the "present" time and vice versa. Each stage has three "Acts" the third of which always takes place in the future, while you start the other two in the present.
The appearance of the future changes depending on the actions you made in the past. Hidden within the past of every level, there is a robot generating machine. If this is destroyed within a zone or all seven time stones are already collected, all of Dr. Robotnik's robots will be destroyed in the past. If you warp into the future, it is a "Good Future" in which there are no enemies and fewer hazards. If the machine is not destroyed, the warp will lead the player into the "Bad Future" in which Dr. Robotnik's robots run rampant, there are more hazards, and heavy pollution and crippled construction has harmed the level.
Music - There is different in each level, and in each time zone. The music is really cool, and catchy. So much you just want listen to it, and it fits the level well.
Overall, Sonic CD is an excellent platformer and really improved the Sonic franchise. If you still have the system to play this, I'd really highly recommend to pick this up. There isn't many good sonic games out there today, but this isn't one of them.