The Sonic CD story arc is a series of strips that, when read together, are a loose adaptation of the Sonic the Hedgehog CD video game. Almost all of the arc concerns the Brotherhood of Metallix and their bid to rule over the Miracle Planet and Mobius (Planet Metallix), although there other activities taken from the game could class as an extension of the arc. Taking this into consideration, the Sonic CD story arc is by far the longest saga in Sonic the Comic, beginning in Issue 21 with the introduction of Amy Rose and has been revisited as late as Issue 255, although it could be argued that Issue 79 was the proper climax.
The first idea to be taken from the game was the introduction of Amy Rose, a female hedgehog from the Emerald Hill Zone claiming to be Sonic the Hedgehog's girlfriend. Troopers arrived to arrest her for this criminal act and took her to the Egg Fortress, the Special Zone base of Doctor Robotnik. Sonic and Johnny Lightfoot arrived to rescue her, but ended up captured themselves by Grimer and the automatic defenses. Robotnik revealed that he planned on turning the Freedom Fighters into Super Badniks that could not be turned back, with Amy being the first subject, but the three escaped before the deed could be done. Not too discouraged, Robotnik and Grimer marvelled at their newest creation - the Metallix!
With Amy now a fugitive, the hedgehog was forced to join the Freedom Fighters and fight for the freedom of Mobius. Unfortunately, this involved her getting kidnapped for a second time, this time by Captain Plunder and the Sky Pirates. The other Freedom Fighters came to rescue her but only Sonic and Tails made it through to the final confrontation. With Sonic and Plunder matching each other in battle, it was Amy who brokered a deal between the two parties; if the heroes inform the pirates of Robotnik ships worth looting, they could help them out in return. True to their word, Plunder came running when Sonic and the gang got themselves captured. Whilst relieving Trusk of all his riches, the Pirates were able to destroy a vast amount of Troopers and freed the Emerald Hill folk from a life of slavery. Unfortunately, Porker Lewis accidentally revealed the location of the Chaos Emeralds to Filch, leading to a final showdown at North Cave. The pirates were successful in stealing the jewels, but the Emeralds actually absorbed their evil, allowing Sonic to easily take them back.
The Brotherhood of Metallix
For full information, see Brotherhood of Metallix.
The arc continued with the introduction of The Brotherhood of Metallixes, several more Metallix models led by the Emperor Metallix. Working from the recently-abandoned Egg Fortress, the band of Metal Sonics managed to create a Pirate Omni-Viewer, forced Grimer to build and re-wrote history so that Robotnik never existed and Mobius was now the steel Planet Metallix. They were eventually destroyed by the same self-destruct mechanism they sought to remove.
Despite their absence, their effects lingered. The Miracle Planet took at least a month to revert to its greener ways. The Pirate Omni-Viewer also remained alive for 15 years, eventually enacting a plan that would have seen the end of Sonic, Robotnik and all time itself had Vector the Crocodile not stepped in.
Unlike the Sonic 3 & Knuckles arc, the Sonic CD arc isn't as clearly defined and can be argued to be a whole different number of stories. Arguably, The Sonic Terminator could be the sole adaptation of the game, fully introducing Metallix and the Miracle Planet. Alongside this are prologue stories Girl Trouble, Pirates of the Mystic Cave and Prologue: Once Upon a Planet.... Stories involving the Metallixes include Count Down to Disaster, Total Chaotix, The Brotherhood of Metallix, The Return of Chaotix, Return to the Miracle Planet and Many Happy Returns, although these may just be part of a wider Metallix arc. Finally, The Seven Badniks introduced the Miracle Planet's Badniks and the Palmtree Panic Zone, but isn't heavily involved with any other story in the arc.
This is the first main story arc to appear in the Sonic universe. The next is the Sonic 3 & Knuckles story arc.