When you start the game off, it'll immediately go to the title screen where you can press start/enter and choose to begin the game, or wait a few seconds to see the nice sparkly anime intro. When you press start, there may be a number of options based on how much of the game you have completed. Press left and right to change the selected option and start to choose it. Pick "New Game" to the far left, to start afresh, or "Continue" to play the game from the start of the last level you played previously, Good Futures and Time Stones withstanding. On the PC and Sonic Gems Collection version, you can also choose to exit the game, on the far right. The game consists of seven levels, each separated into three segments. Ordinarily, these segments would be called "acts", but in Sonic CD, rather confusingly, they are called "zones", which is a word usually reserved for the level as a whole. The first two zones contain the usual running through curved roads, jumping on platforms and bopping enemies that you would expect from any proper Sonic game, but the third zone is mainly a boss fight, with only a little of the usual platforming going on, leading up to it. I will get to the game's exclusive time travel aspect in a moment.

Controls for Sonic are the same as the Mega Drive games, although Sonic feels a little stiffer to move around now. Move him left and right with the left and right buttons on the D-pad (or the keyboard's arrow keys for the PC version), and he'll gradually increase his speed the longer you hold it. Press up or down to make Sonic look up/crouch down and tap either button twice quickly, and hold it on the second time to make the screen scroll up or down a little way, so that you can see your surroundings. A, B and C (or Spacebar, for the PC version) are all jump, and can be used for the following maneuvers:

Press A B or C to launch straight into a spinning jump. As a spinning ball, you'll destroy enemy robots and barriers, and open up item boxes on contact. You'll stop spinning as soon as you land back on the ground, and the height of the jump is dependant on how long you hold down the button for when you press it.

While moving along the ground, press down to curl in a ball and perform the rolling version of the spin attack. This can take out enemies on the ground and is best used when heading down a slope, as your speed will greatly increase. Jump to get out of it, or just wait until you lose all momentum.

The Spin Dash debuted with Sonic 2, which was made at the same time. It's here, but it works a bit differently to the norm, and you get the impression it was thrown in towards the end of production. When you're not moving, hold the down button and Sonic will crouch down. Then press A B or C while still holding down and he'll begin to spin on the spot. Hold down for at least a couple of seconds, and then release it to let Sonic go spinning fairly quickly across the ground in the direction you were facing, taking out any enemies in his way. If you let go before those first couple of seconds, it won't work and Sonic will just remain where he is, unlike in every other game where there is no mandatory amount of charge up time. You can press A, B or C or hold down for as much as you like, but it doesn't seem to make any difference to how quickly or far he is likely to go, either.

Basically the running version of the Spin Dash. Hold up instead of down then press A, B or C, and Sonic will begin to run on the spot. Keep holding up until you see Sonic at top speed (that weird "figure 8" shape he makes with his legs that seems to only be in this game), which should take about a second, and then release up to blast away across the path. Of course in this version, Sonic is not spinning, so he will be vulnerable to running into a harmful obstacle or enemy. However, at least with the peel-out, you have a visual cue as to when you can release the button, unlike the spin dash.

The thing that really makes Sonic CD special is the time travelling feature, which thus far has not been found in any other Sonic game. Every level has four different parallel versions to it, across three different time periods, and you can travel between them as you progress through the level. In the first two zones of each of the seven levels, there's a Past, a Present, and two possible Futures - good and bad. The third zone (the boss) is set only in the Future, so there are no Past or Present versions there. In each of these timezones, the visual appearance changes dramatically, and the level structure and object placements may differ to varying degrees too. The Present is where you automatically begin the first two zones of each level, and from there you can either travel to the Past or Future using past or future signposts, as explained in the items section below. The Past represents how the level used to look, and Eggman has only begun to place his machinery. The Bad Future is an outcome where Eggman has completely taken over and the whole place is a mass of gloomy technology. By contrast, the alternate Good Future is nice and happy. Any trace of the mad scientist is long gone and the resident animals are free to frolic in a utopian world, though this is still usually mechanical in some aspect.

So what is the point of all this then? Well, if you're just looking for a simple Sonic platforming experience, you're quite free to ignore all of this time travelling malarky and just leg it through all seven levels, no fuss. This would be missing the point of the game though, and there would only be Bad Futures. The main objective is to save Little Planet by undoing all of Eggman's work and aiming to change them into Good Futures for every level. To do this, in the first two zones of each level, you need to travel to the Past and locate a special machine that Eggman has hidden somewhere. By destroying this machine, you earn a Good Future for that zone, and if you wish, you can then travel to the Future before completing the zone, to witness the peace that you've brought to it. This is not necessary though, you just have to destroy the machine to change the Future, otherwise it will remain as the Bad Future. The third, boss zone of each level can only be set in the Future, but which one it is depends on whether or not you destroyed the machines in the Past versions of BOTH of the first two zones. If you only did one of them, you do not get a Good Future for Zone 3, which is what counts, as you have to beat the third zone in the Good Future to completely secure the level. The alternate method of creating Good Futures is to collect all seven of the Time Stones, found in the Special Stages. With all of these in tow, every Future is automatically turned good when you start a level.

Sonic's life support system. They're scattered around everywhere, but as long as you have at least one, you won't die when you take a hit from a badnik or obstacle (unless you get crushed by something). Instead, you'll lose all the rings you have, and they'll scatter around the place. Sonic will then be flashing for a couple of seconds, to ensure that he doesn't get hit again until you've had a chance to move him away or grab a lost ring or two. However, if you take a hit without having any rings (or a protective shield), you'll lose a life. Collect 100 rings to gain an extra life, and each one that you're holding at the end of the zone is worth 100 bonus points for your score. Most rings transcend the timezones, so when you see some in the ground that are inaccessible, you'll be able to grab them in a different timezone, where that ground won't be there.

Use your spinning ability to open up these item boxes. Depending on the picture, you'll either get 10 extra rings, an extra life, a blue shield, a speed boost or invincibility. The blue shield surrounds Sonic, and when he takes a hit, he'll lose the shield, rather than any rings. The speed boost lasts for 20 seconds and will increase the speed of Sonic's movement, and change the music to the speed-up theme. Similarly, an invincibility power-up will surround him in small stars for the next 20 seconds, during which time he'll be invulnerable to any attack (except falling off the screen, drowning or getting crushed). Any badniks that you touch will also explode, and the music is changed to the invincibility theme.

Also known as point marker, starpost, etc, throughout the series. Touch one when it's blue and your position is saved so that if you die afterwards, you'll return to that place, rather than starting from the beginning of the zone. They flash red and yellow to indicate that you've already touched them, and only appear, and therefore are only valid, when in the Present. Past and Futures do not have any checkpoints, so if you die in any timezone other than the Present, you'll always return to the beginning of the zone, but still in that timezone.

These are your only method of moving between the different timezones, and using them can be a little tricky, so listen up. They are located in various places across the zones, and will either say "Past" or "Future" on them (both are found in the Present but only past posts can be found in the Future and only future posts can be found in the Past). Touch one to get it spinning, which will prevent it from being used again for the rest of your time spent in the zone (though losing a life will reset them all), and a past/future panel will be added to the lower left-hand corner of the screen. In order to use it, you basically have to run fast enough that you maintain a top speed for 3-5 seconds, without it being interrupted by running into a wall or obstacle. Just like in Back to the Future.. sorta. When you reach top speed while having a past/future panel activated, a trail of white stars will follow Sonic as he runs or spins along, and these will automatically kill any badnik you happen to collide with too, which is handy. If you mess up during the first couple of seconds of getting the white stars, the past/future panel will still be active, so you can try again. During the last 1 or 2 seconds though, the panel will begin to flash and if you mess up, it'll disappear and you'll need to find another signpost that you haven't already touched. Maintain your speed for the full amount of seconds though, and Sonic will be launched into the Past or Future, as the screen goes white and he he blasts upwards through a psychedelic flashing background, reappearing in the same place, but in the new timezone.

It can be hard to maintain top speed just by running through the level, but fortunately there are quite a few structural designs or mechanisms that you can take advantage of, which will keep you going without interruption. Note that this procedure will send you only one place forward or back in time, so that going to the Future while in the Past will return you to the Present, and not straight to the Future, and vice versa. Also note that the exact amount of seconds required to time travel appears to vary between the original Mega CD version and the PC/Sonic Gems Collection version. In the Mega CD version, time travel takes about 5 seconds of top speed, while on the PC (and subsequently the PC port featured on Gems Collection), it only takes about 3, which makes things much easier.

This, ladies and gentlemen, is your target. In the Past of every first and second zone of every level you will find one of these machines hidden away somewhere. All you have to do is destroy it by jumping on it, and you'll immediately ensure a Good Future for that zone, even if you then die before getting to the end (unless you lose all your lives). It's often surrounded by enemies, but every enemy in the zone automatically turns to dust as soon as it's destroyed. You can see what remains of the machine in the Present, in the same place, but you can only destroy it when in the Past. They are also worth 1500 additional points.

Something else to locate and destroy, though this is an entirely optional extra. In the Past of every Zone 1 and 2 (except for the final level), you'll find a hologram of Metal Sonic causing trouble amongst the local animal residents. Destroy the hologram generator next to him and the animals will begin to prance around the stage freely, as they do in the Good Future. That's your good deed for the day.

This is nothing more than a visual guide to tell you that the end of the zone is just to the right of its location. This can be very useful if you want Good Futures and are trying to locate the machine in the Past, or earn enough rings to qualify for the Special Stage, because if you get too close to the end-of-zone signpost, the screen will close behind you and won't let you go back. This goal signpost gives you a warning and as long as you stay to the left of it, you'll be able to continue your play in the zone, until you're ready to complete it.

This signpost showing Dr. Robotnik's ugly mug can be found at the end of every Zone 1 and 2. Touch it to get it spinning, where it'll eventually land on a picture of Sonic's face, and bring up the zone completion procedure, where your score is tallied. This is the only way to end the zone, and continue to the next one, and Sonic will automatically walk off-screen to the right after a moment.

If you manage to complete any Zone 1 or 2 with 50 rings on hand, a huge ring can be found to the right of the end-of-zone signpost. Jump in it to access one of the seven Special Stages, where you'll get an opportunity to obtain one Time Stone. See the Special Stage section for details.

Come in horizontal, vertical and diagonal flavours, and this game can often be quite imaginative with how it uses them, in some levels. The verticals and diagonals are used to send Sonic flying upwards or across when he jumps on or touches them, and you have a certain amount of directional control, using the left and right buttons. You will not be spinning, however, so be careful of badniks and other obstacles along the way. The horizontals can be used as a quick speed boost across the ground, and red springs are more powerful than yellow ones. Some vertical springs are embedded in the ground, and while normally a pair of horizontals that face each other would be nothing more than an annoyance, in this game, they're extremely useful for zipping between timezones easily. Another common use for a spring is to have it on a little wheel, cycling back and forth over a small area.

Each small set consists of four spikes, and are clearly harmful to touch on their business end, but their sides are safe. Can be found on the ground, ceiling, or horizontally on the walls, and in either large groups or single sets, to jump over. Some can repeatedly slot in and out of the ground.

Flat grey switches in some levels can cause something nearby to change when you land on top of them. Often for opening up doorways or bridges.

Only found in one level of this game, but I thought I better mention them here anyway. For sections where you have to go underwater, Sonic can last only 30 seconds without air. During the final 10 seconds, a countdown will begin, and you'll drown at the end of it if you don't get to the surface, or obtain some air from these bubble clusters, which are found at various points along the ground. They produce small bubbles and big bubbles, and its the big ones that give you another 30 seconds worth of air, although you may have to wait a little bit before they are produced. Just touch them to suck 'em up. Unlike in most other games of the series, there is no scary countdown theme. Only a series of bubbly sound effects.