Shadow the Hedgehog was released in late 2005 for the Nintendo Gamecube, Playstation 2 and the original Xbox. It was developed by Sega Studios USA (originally known as Sonic Team USA), an American division of Sonic Team that had previously developed Sonic Adventure 2 and Sonic Heroes. After the release of Shadow the Hedgehog development of mainline Sonic games was handed back to Sonic Team proper.
As far as story is concerned, Shadow the Hedgehog follows on from Sonic Heroes. At the end of that game, Shadow found out that Dr. Eggman had been building androids that looked like exact copies of Shadow himself. Seeing as Shadow had no memories of his past this caused him to begin doubting his own existence. Was he the original Shadow the Hedgehog created by Dr. Gerald Robotnik 50 years prior, or was he merely one of Dr. Eggman's assembly line androids?
At the start of this game, Shadow's search for the truth is interrupted when an army of aliens called The Black Arms invades the planet. Their leadercalled Black Doomclaims to have made a pact with Shadow to retrieve the Chaos Emeralds in the past. Shadow, being the hardboiled lone ranger that he is, decides to use this as an opportunity to find out the truth about himself. Whether he gets it by trusting Black Doom and assisting him in conquering the planet or by helping the military put an end to the alien invasion is for the player to decide.
Shadow the Hedgehog is a very bad game. It looks about as good and plays about as well as an early licensed PS2 game. The controls are at once sluggish and oversensitive, level design elements are endlessly cut and pasted (even across different levels), objectives are confusing and often difficult to find, cutscenes and voice acting are all incredibly poorly executed, the list goes on. Even the positive aspects of other bad 3D Sonic games like a decent soundtrack and interesting locales are nowhere to be found here. The game has almost no redeeming qualities which is what makes it such an enticing trainwreck.
On top of being poorly executed the game is also rather ill-conceived to begin with. It takes the relatively light-hearted world of Sonic the Hedgehog and tries to make it more "mature" in the worst possible ways. Characters say "damn" and "hell" all the time, the music consists mostly of grungy mood rock, the colour palette is largely limited to dark and smudgy colours and the central villain looks and sounds like something out of Metalapocalyse. On top of all that, Shadow gets honest-to-goodness firearms to fight enemies with. We're talking pistols, assault rifles and even grenade launchers. You still have access to the signature spindash and homing attacks but the guns are simply more efficient combat tools. The game also constantly urges you to use vehicles like jeeps and motorcycles to get around faster. Oh and also Shadow goes around killing human soldiers with absolutely no remorse.
Long story short, I wish they would've made Billy Hatcher 2 instead.