Released towards the end of 1993 for the Sega Master System (in Europe and Brazil only) and Game Gear, Sonic Chaos—or Sonic & Tails as it's called in Japanesenote —makes quite a few improvements upon its predecessor, the 8-bit version of Sonic the Hedgehog 2.
One year has passed since the six Chaos Emeralds were restored to South Island, and there was much peace and quiet (except for the occasional sonic boom). Then one day, an explosion was felt throughout the island, transforming the terrain in strange ways, and the animal inhabitants searched for the cause. Flicky the Bluebird discovered that the Chaos Emeralds were missing from the North Cave, and Miles "Tails" Prower guessed that the separation of the Emeralds will cause an imbalance of chaos in the world and even lead to South Island sinking into the ocean. Not a moment after that grave deduction, Dr. Robotnik (aka Dr. Eggman) appeared in his signature mobile, the pincers underneath clutching the red Chaos Emerald! After gloating about his plot to Take Over the World and making his getaway, Sonic the Hedgehog resolved to race to the other five Emeralds before it's too late. With six zones, three acts each, and five Emeralds to collect, it's time to hero up and save South Island!
Sonic Chaos presents a couple of firsts for 8-bit Sonic title. Notably, it's the first where the Chaos Emeralds are collected via Special Stage instead of in the actual zones, the first to feature the Spin Dash, and also the first to allow you to play as Tails.
But in a first for the series as a whole, Sonic and Tails were differentiated gameplay-wise. Sonic can perform the Peel Out/Strike Dash - a technique carried over from Sonic the Hedgehog CD - and for some reason he's the only one who can enter Special Stages (by collecting 100 Rings in a regular act). Tails is slower, but he can fly, and he has the benefit of starting the game with a couple of continues (Sonic has to work for his!). Needless to say, Tails' game is considered the easier of the two.
Items introduced here were the Rocket Shoes, which sends your character forward with a powerful thrust of speed and mild flight capabilities for a short period of time, and the Hop Springs, which allows you to do extra high jumps but also for a short period of time.
This game provides examples of:
- Acrofatic: Dr. Robotnik managed to keep pace with Sonic and Tails during his final escape sequence.
- Added Alliterative Appeal: Electric Egg.
- All There in the Manual: The Japanese manual provides the names of all the bosses, as well as descriptions of each zone. Some are what you'd expect(eg. Mecha Green Hill Zone: Green Hill but mechanized by Eggman) and some not (eg. Aqua Planet: a literal planet in another dimension).
- Bonus Stage: The Special Stages. Only Sonic can access these stages, and that's by collecting 100 rings.
- Boss Corridor: Past Turquoise Hill, every Master Robot has a corridor separating it from the platforming part of Act 3.
- Boss-Only Level: Again, Act 3 of each Zone has a small platforming pre-boss area, some sort of straightaway, and then the Master Robot. This time, there are rings and sometimes extra lives and invincibility boxes to the act to make the fights easier. The rings are off the beaten path in the last three Act 3s, however.
- Bottomless Pit: Unlike the previous game (which only had these pits in Sky High Zone Act 3 and otherwise used spike pits), there are multiple bottomless pits in Sonic Chaos starting with Sleeping Egg Zone, and you get two of them in the final act of the game.
- Chicken Walker: A "Laser Walker" on skinny legs is the machine Robotnik is operating when Sonic/Tails meets him at the end of the game (it's even called a "Killer Turkey" unit in the Japanese manual). It can jump, crouch, and fire two kinds of shots (including a ricochet shot).
- Cryptic Background Reference: In the original Master System manual, the North Cave is briefly mentioned as a gathering place or hiding spot of the six previously collected Chaos Emeralds, but the concept is then dropped entirely from the series. However, in Sonic the Comic, it is explained as location in the Frozen Zone once used to freeze the Chaos Emeralds and prevent them from wandering aimlessly due to their natural instability.
- Cyber Space: Electric Egg Zone, which has a computer board design to it. Sonic Chaos is the first handheld Sonic game to use a Cyber Space level for a Robotnik-themed stage, something that Sonic Advance 2 & 3 would adapt for their semi-final full-length zones.
- Dub Name Change: While the Master System manual leaves them unnamed (leaving the job to a series of Sonic the Comic Q Zones), the Game Gear manual names most of the Badniks and Master Robots, and they all differ between the English and Japanese versions. Additionally, the main antagonist is once again known as Dr. Robotnik in western versions and Dr. Eggman in Japanese versions, although the original Master System still lists his name as Robotnik in the cast roll (justified since it didn't even reach Japan until the Wii Virtual Console release in 2009).
- Eternal Engine: Electric Egg Zone and part of the design for Mecha Green Hill.
- Final Boss: Like the 8-bit version of Sonic the Hedgehog 2, you only face Robotnik's Master Robot minions as the bosses of the Zones prior to Electric Egg Zone. Robotnik himself still fills the Final Boss role and is still fought in his own walker mech at the end of the last Zone.
- Floating Continent: Sleeping Egg is surprisingly this. According to the Japanese manual, it is described as a "mysterious island floating in the sky". As evident by the murals of the vile doctor, it is "actually a fantasy world created by Eggman".
- Flunky Boss: The boss of Aqua Planet Zone first sends 6 hopping bird mechs at you. The player must destroy all of them to make the boss appear. It also uses bird mechs as missiles in its last-ditch attempt to defeat Sonic/Tails.
- Go for the Eye: When confronting the tree hugger Master Robot in Mecha Green Hill, one must land on his exposed eyes on the top of the machine to damage it.
- The Goomba: Moto Bug-style Badniks return in this game, but now have Springs mounted on their backs, meaning you can only hurt them by rolling into them. They are called "Bane (Spring) Motora" to make this clear in the Japanese manual, but the English versions kind of botch the connection by calling them Spring Snails and Boing-o-Bots. A giant variation of them (called the "Ladybug Boss" in the Game Gear manual) appears as the first boss of the game in a vein similar to Egg Mobile-D. Variants of Buzz Bomber also recur in this game, but only have their stingers so they're more Buzzes and less Bombers.
- Green Hill Zone: Turquoise Hill Zone and Mecha Green Hill Zone.
- Hailfire Peaks: Mecha Green Hill Zone combines Green Hill Zone with Eternal Engine, resulting in a once beautiful tropical paradise industrialized in the ugliest way possible.
- Hopping Machine: The Pogo-Powered Robotnik (MS)/Hopping Egg Volkan (JPN GG)/Bouncy Boss Robot (ENG GG) from Sleeping Egg Zone.
- In Case of Boss Fight, Break Glass: The pogo-powered Robotnik Master Robot in Sleeping Egg and Robotnik himself in Electric Egg require the player to jump on their cockpits to destroy them.
- Inconsistent Dub: Unusually, Sonic Chaos has a reworked storyline (and mostly manual text, for that matter) between the Master System and Game Gear instruction booklets. This is the only 8-bit title that does so. If one includes the Sonic & Tails version, that's three major variations.
- Law of 100: How you get into the Special Stages. You grab 100 rings in a non-boss act, and you're automatically taken to them. After you gain the 5 Chaos Emeralds in the Special Stages, grabbing 100 rings awards you an extra life like it does in Tails' playthrough.
- Level Goal: There's a signpost at the end of each normal act as usual, but interestingly enough the game is instead designed around exploring each level until you get 100 rings to enter a Special Stage. Doing this also counts as clearing the level and sends you on the track to the good "ending".
- Made of Explodium: The explosive "Coconut-Bots" in Mecha Green Hill Zone (bombs attached to several of the coconut trees in the Zone that fall and explode on the ground when you pass under the tree. They always come two at a time).
- Multiple Endings: As with most Sonic games, collecting all the Chaos Emeralds results in a good ending; although there are only five Special Stages, the sixth Chaos Emerald in Robotnik's clutches will automatically be reclaimed if the others are found. However, if you're playing as Tails, then...
- No Ending: Even when playing as Sonic, and clearing the game with all emeralds, all you get is a slightly different credits sequence.
- One-Hit KO: The second part of the Final Boss, which will kill the player in one hit even with rings on hand. Thankfully, it works the other way around.
- Palmtree Panic: Turquoise Hill Zone.
- Rearrange the Song:
- Recurring Riff: Mecha Green Hill Zone's theme is You Can Do Anything. It's also part of the Final Boss theme and invincibility theme.
- Rocket-Tag Gameplay: The very last challenge in the game boils down to this (see One-Hit KO).
- Sequential Boss: The last two bosses fit this:
- The Master Robot in Aqua Planet Zone Act 3 first sends six Mecha Hiyoko chick badniks at Sonic/Tails (these are the same enemies from the other two acts, but they each require two hits). Once they are destroyed, the boss will drop down and start jumping and firing bullets, and Sonic/Tails must bounce off the top to damage it. Once it blows up, it pulls a last-ditch attack by sending bird missiles at Sonic/Tails. Once it reappears, it takes one more hit to destroy it and finish the level.
- The final fight with Robotnik at the end of Electric Egg Zone Act 3 has two phases. The first pits you against the walker with Robotnik in it, and unlike most other boss vehicles up to this point, Sonic/Tails has to specifically hit Robotnik and his cockpit directly to damage the machine. After it blows up, Robotnik retreats a few paces and converts what's left of his mobile into a speed ship and tries to ram the player for a One-Hit Kill. This is sudden death for him too; all it takes is one additional hit on him to down the craft and send Robotnik scurrying for the escape jet for his Villain: Exit, Stage Left to end the game.
- Shout-Out: The level select (Up, Up, Down, Down, Right, Left, Right, Left, Start at the title screen) and sound test (Down, Down, Up, Up, Left, Right, Left, Right, 1, 2, Start at the title screen) codes are both variations on the Konami Code.
- In the sound test menu, if you perform a code, Sonic will pull off a Hadouken.
- Spell My Name with an "S": Gigalopolis Zone was changed to Gigapolis Zone on international versions of the Game Gear ports due to the change in font so that the name can fit on the screen.
- Spring Coil: Hop Spring (MS)/Hopping (JPN GG)/Pogo Springs (ENG GG).
- Tomorrow Land: Gigalopolis Zone (MS/JPN GG)/Gigapolis Zone (ENG GG).
- Underwater Ruins: Aqua Planet Zone which, while commonly believed to be directly on South Island, is actually a "water planet in Another Dimension" according to the Japanese manual. Though what dimension specifically or how this phenomenon came to be is never mentioned. Regardless, it is somehow tied to the Chaos Emeralds and Planet Mobius/the earth/Sonic's world.
- Villain: Exit, Stage Left: After defeating Robotnik at the end of Electric Egg Zone Act 3, he outruns Sonic/Tails to an escape platform and flees. On Sonic, if you got all 5 of the Chaos Emeralds, Robotnik drops the 6th on his way out and you can pick it up to end the game. Otherwise, Sonic/Tails leave off the right of the screen like the other zones, but without the extended run.
- Wake-Up Call Boss: The pogo-powered Robotnik Master Robot at the end of Sleeping Egg Zone Act 3 bounces and rapid-fires bullets that explode when they hit the ground. Unless Sonic/Tails is bouncing on the machine directly, dodging the bullets can be a giant pain and tell you the game's not as simple as you may have been led to believe by that point.
- Warm-Up Boss: The Tracked Bug (MS)/Great Bane Motora Gold (JPN GG)/Lady Bug Boss (ENG GG) from Turquoise Hill Zone. It can't even directly harm you—it's just a Spring Snail, but bigger and with hit points. Its weak spot is its front, which you can ram with the Spin Dash, bounce off of, and rinse and repeat to destroy in roughly ten seconds.