The plot beats of Sonic Forces are a bit different than the campy affairs of most Sonic games. Dr Eggman has already conquered the world, so instead of trying to stop his crazy schemes, the Sonics and your avatar are leading the resistance to try and overthrow the tyrant. Many people were wondering why Sega went with a darker plot, and Shun Nakamura of SEGA gives more enlightenment via a Game Informer interview on this tonal change.
Here’s Nakamura on the darker story:
With Sonic Forces, you’re taking a decidedly darker approach than we’ve previously seen; Dr. Eggman has already taken over and Sonic is leading the resistance. Was this an intentional shift after recent games were more lighthearted in tone"
It wasn?t necessarily that we set out to make a dark world, but when we were thinking about making everything a little more dramatic and making the enemies feel [more like] enemies, what the team really wanted to do was have Sonic as this really powerful, positive character and then have the enemies as these really strong villain characters. What we wanted to do was really make those villain characters feel like villains and to do that we need to see some villainous activities going on ? there?s got to be something done to this world to make it feel like Eggman is really taking over and is a threat. By putting Sonic in this threatening situation, it turned out that the world had to become dark and we had to tell this dark story. Additionally, a...