Sonic the Hedgehog writer Ian Flynn has talked to IGN
Sonic Frontiers general story:
“Sonic, Tails and Amy embark for the Starfall Islands to investigate the disappearance of the Chaos Emeralds. Things take a turn for the dramatic immediately, and Sonic is left alone with a lot of questions. The story is his journey to uncover all the mysteries of the islands, rescue his friends, and discover how everything ties together.”
Sonic Frontiers theme:
“Melancholy” is a good word for the story overall. Sonic’s indomitable spirit carries him throughout the adventure, and it’s what allows him to help each of his friends as they tackle their own personal challenges. Someone else you encounter will have their very identity shaped by their interactions with Sonic, which isn’t always a comfortable process. Then there’s the secrets of the Starfall Islands themselves and how the tragedies of the past have led to the adventure today. Sonic’s “never say die” attitude is certainly put to the test.”