This game is set one year after Sonic Mania's bad ending where Robotnik had the Phantom Ruby and Chaos Emeralds in possession. After the final battle in the core of the Titanic Monarch, Robotnik was stuck in Little Planet and started collecting the Time Stones and preparing his greatest plan yet. In the present, Sonic decided to invite Tails and Knuckles (but Knuckles rejects the invitation to guard the Master Emerald) to vacations on Heaven Shore, a pleasant beach he loves to relax in. Suddenly, Sonic witnesses Amy, Mighty, and Ray being kidnapped by Robotnik but the second Sonic was going to hit Robotnik, a fast creature flew into Sonic with Knuckles chasing it. Turns out this was Meta, a cybernetic fennec fox who is working for Robotnik. Meta had stolen the Master Emerald. Sonic, Tails, and Knuckles agreed to take separate paths: Sonic will retrieve the Time Stones, Tails will save Amy, Mighty, and Ray, and Knuckles will retrieve the Master Emerald. Soon, they will discover a third foe, Fang the Sniper, who works with Robotnik as a mercenary. Also yes, Robotnik made all that to separate Sonic, Tails, and Knuckles.
