After going through a portal in Mobius to catch Dr. Eggman, Sonic gets transported to Jump City, where the Teen Titans live on Earth. After getting help from Robin, Starfire, Cyborg, Beast Boy, and Raven, they come across the rest of Sonic's friends and a terrible surprise. The long lost Slade has teamed up with enemies of Sonic, making a team that could be invincible. Will Sonic and the Teen Titans be able to stop Slade and his allies from taking over the world?

Prologue pretty much says it all

After Frisk goes missing, Caitlin, Frisk's twin sister goes looking for her in the the last place she saw her. MT Ebott, the mountain of dark legends, she finds a necklace that belonged to Frisk on the ground. Just like Frisk, Caitlin tripped and fell into the Underground. Caitlin finds things she never knew existed.