Chapter 1~The Ultimatum

Same storyline as Sonic Underground except for one detail. They're not triplets, they're quadruplets!

Dr. Robotnik has captured their youngest sister, Ambrosia, Amber for short. Using her as bait, he informs them that at the end of the week, he'll Robotisize her. Terrified at the prospect of losing their sister before they've even found their mother; the three siblings come up with a rescue plan...