In 1993, Sega released a new Sonic title for the Mega CD add-on, named Sonic the Hedgehog CD or Sonic CD, for short. It's considered widely to be one of the few extremely worthwhile games available for the system, and there are even some fans who consider this to be the best Sonic game ever made.
I presume the story takes place between the events of Sonic 1 and Sonic 2, though I could be wrong about that so take that with a grain of salt. Sonic arrives at a place called Never Lake, where a mysterious small planet, named Little Planet, appears over said lake annually. Upon arriving, he sees that it's been chained down, thanks to Dr. Robotnik who is once again up to his old tricks. To make matters a little more complicated, Sonic has been followed to Little Planet by a young pink hedgehog named Amy Rose, who is utterly infatuated with Sonic, and not long later, she is kidnapped by Robotnik's newest robot, Metal Sonic. The beginning and end of the story is told through animated cutscenes which are rather nice to look at, if a little grainy.
From a gameplay perspective, it's Sonic. You still run, jump, roll into Badniks, collect Rings to stay alive, TV monitors, get to the end of the stage, it's all here. Though it's worth noting that when you smash Badniks, flowers come out of them instead of animals.
There are a few significant differences, though: Sonic CD was being developed at the same time as Sonic 2, so a lot of the features either didn't make it in or seem different. The spindash was one of the first things I noticed was different; instead of simply holding down and tapping the jump button, you need to hold both down and the jump button for a set period of a few seconds. Let go too early, and you're going nowhere. It goes without saying that the other spindash is much better; this one is pretty slow so I barely ever use it. Thankfully, the game also introduced a new move called the Super Peel-Out, where the wind up time is so much shorter, but you're not in a ball form so you will get hit by enemies or stage hazards if you're not careful.
The goal of the game is to stop Robotnik's plans, but there's a little more to it than simply going to the end of the game and beating him down. Robotnik's taken over Little Planet by travelling back to the past and putting a roboticizor there. Therefore Sonic too will need to travel back to the past in order to destroy said roboticizor in order to ensure the planet has a good future. Getting all good futures will enable you to get the best ending. Whether that matters or not, is up to each individual.
Sonic will find posts scattered in each act, some with the word past, and some with the word future. The future ones are pointless; they take you to the future which doesn't serve much of a purpose in the long run bar from allowing you to see what could be depending on whether you ensure a good future or not. The past ones, however, are the ones you need to take you back in time to destroy the roboticizor. When you find and pass a post, you need to get up to and keep a consistently fast speed. This may sound relatively simple, but if that sign starts flashing and you slow down even a little, you're not going anywhere. This can be tricky if you don't have any idea of the level layout and which areas will let you get up to and maintain speed, not to mention there is the odd collision detection problem that can also cut your speed prematurely.
Levels, of which are there are 7 zones with 2 acts each, are a bit more platform-heavy than in other mainstream classic Sonic titles, which means that maintaining a high speed isn't always easy. In fact I feel a lot of the level design isn't all that great compared to other Sonic games anyway. Then you have to find the roboticizors, which are hidden somewhere in the level. There's a lot more emphasis on exploration in Sonic CD than there ever was in previous Sonic games at that point. That's not a bad thing, but it can become tedious for some, particularly as, unlike Sonic 3 & Knuckles, time travelling is something you must do in order to get roboticizors.
Don't fancy that? Well, fear not, for there is another way to get a good ending for those that don't feel like going after the roboticizors and that is through the special stages. Simply collect 50 Rings by the end of the level just like in Sonic 1. This will take you, once again, to a special stage. In these special stages, you're in a rather nice looking 3D environment - though occasionally really trippy sometimes - running around destroying all of the UFOs in the environment. Getting all of the UFOs will grant you a Time Stone and they don't grant Sonic any hidden abilities, just the good ending and nothing more. It's not as easy as it sounds as there are a few factors that can make these quite challenging. For one, touching water causes your timer to go down insanely fast. Secondly, the special stages do have a few lag issues. Thirdly, in later special stages, the UFOs sometimes like to turn right as your jumping into them. Still, there is a blue UFO that turns up when your timer is running low to give you more time if you catch it. That's something I guess. I don't consider these to be my favourite special stages, not by a long shot, but I do enjoy them regardless of my complaints and I do really appreciate the fact that they tried something different here.
Once again there are bosses to fight at the end of each zone, though it has it's own separate act for that this time instead of it being at the end of act 2. The boss fights are admittedly really disappointing due to being very easy. Though with that being said, the bosses are really unique and the means of beating them often isn't as simple as bopping them on the head.
Graphically, the game is colourful. Some people may argue it's too colourful. I like the details and the look of each level and in all honesty, the bright colours never bothered me all that much, not even when the stage goes super fast. As for the soundtrack, it is pretty amazing. There were two different soundtracks released; the Japanese version, which Europe also got, and the American version. Many people constantly debate over which is the best soundtrack, but in all honesty, I like both. Everybody has their preferences on the soundtrack, but in my mind, both are good and well worth listening to.
Sonic CD is considered by many to be the best Sonic game ever made. I personally think that it's a decent game, and I get a kick out of it if I play through the game without bothering to get the good ending but when I do try and get the good ending, it quickly becomes a lot more tedious to me. It can be a rather enjoyable game, but I just think the other classic Sonic titles are so much better. If you want to play Sonic CD, I recommend the HD port released on Xbox 360, PS3 or PC, as it features the option to choose which soundtrack you want to listen to, has a second playable character in the form of Tails and fixes a lot of the collision detection and lag issues the original had.