Sonic 2D All-Stars is a collection of 2D remakes of three of Sonic's 3D outings, plus a fourth game that reimagines how Silver, Blaze, and Marine first met each other as well as Sonic and friends. Taking place in a continuity alternate from the original one, they are each renamed after their respective storylines, including the Chaos Saga (Sonic Adventure 1's rewritten story), the ARK Saga (SA2's storyline), the Black Doom Saga (Shadow the Hedgehog's storyline), and the Eggman Nega Saga (Sonic Rush, with some bits of Sonic Riders and Sonic '06). The games were made in responses to the criticisms from fans and critics alike, who said that the stories weren't good enough, the characters felt added in for the sake of being added in, and that Sonic doesn't translate into 3D very well.
Gameplay is reminiscent of the original 2D Sonics for the Genesis and Sega CD, where the player controls their chosen character(s) through a stage, collecting rings, dodging obstacles, destroying enemies, and eventually fighting an end-boss. Because they're in 2D, they have almost none of the problems that made the 3D games so infamous today. As for the continuity errors, in order for them to be fixed, an alternate continuity had to be made from scratch, so the games' stories would be rewritten with a lot of the technicalities trimmed-down, and the characters' personal-development could be more fleshed-out.
This details the rewritten events of Sonic Adventure 1, complete with Rouge the bat and Cream the rabbit added in the mix as playable characters besides Sonic himself. Tails, Knuckles, Amy, Big, E-102 Gamma, and the rest of the cast aren't playable.
This details the rewritten events of Sonic Adventure 2, complete with Cream the rabbit added in the mix as a playable character besides Sonic, Rouge, and Shadow. The others aren't playable, and it comes with an alternate ending wholly different from the original version's.
This chronicles the reimagined events of Shadow the Hedgehog, with Sonic as a playable character. Players that select Sonic rather than Shadow will see through his and his friends' perspectives, as they are forced to decide whether to redeem Shadow at the risk of being hunted down by the corrupt GUN Army for sympathizing with him, kill him so that GUN wouldn't hunt them down, or take the neutral route and collect the Chaos Emeralds before Black Doom and his alien army collects them first. This game utilizes Sonic Advance 3's tag-action system, where players gain different abilities depending on which AI-controlled partner they select, and the selectable partners change or get added in or omitted depending on the stage and choices the player selects.
This chronicles the alternate events of Sonic Rush, with Silver and Marine hailing from another dimension like Blaze the cat, rather than either in the future or some other random place. Sonic and Blaze are both playable characters, although Silver, Marine, and the rest of the NPC-cast aren't. Much like the Black Doom Saga, depending on which level each character is on, players gain one of several different sets of AI-controlled tag-team partners to pick from, each giving the player different abilities and tag-team actions to access different paths they'd otherwise wouldn't access, similar to Sonic Advance 3.