See part 1 on the Steam Wiki if you haven't!
Steam Dimensions is inspired by Lego Dimensions.
After clearing "Green Hill Zone act 1" (and the bonus area, of course) Sonic and Tails got the first Chaos Emerald. But then, "Green Hill Zone act 2" was over-filled with Badniks.
Sonic: This doesn't look too bad, right Tails...? Tails?
Only to notice that Tails is getting sucked up by a rift! Along with the Chaos Emerald!
Tails: SONIC!!!
Sonic immediately went straight to the rift, at the speed of sound.
Dr. Eggman: Aww... I had it prepared!
Continued at Undertale AU Wiki.
Episode 1 - The Foundation Altar
Episode 2 - Chaos Emerald Snatched!
Episode 3 - The Spaghetti Festival
Episode 4 - Inkwell Dance-Off!
Episode 5 - The Portal
Episode 6 - Prop Mayhem!
Episode 7 - Our first Keystone
Episode 8 - Vs Machines!
Episode 9 - A Familiar face
Episode 10 - I'm your Neighbour!
Episode 11 - 6 a.m.
Episode 12 - Collided Hill Zone
Episode 13 - Roller Coaster Mayhem
Episode 14 - Aperture Laboratories
Episode 15 - Zombies!
Episode 16 - Bloon Invasion!
Episode 17 - Geometric World
Episode 18 - Deadlocked!
Episode 19 - Steam
Episode 20 - Collision of Dimensions
Episode 21 - Getting Help
Episode 22 - Showdown Time!