Here is the story:

An alternate universe parallel to Sonic Forces on Earth, Christmas Island. Dr. Eggman build plans to create all-new world, which will be called Eggmanland and will belong only to him. When he tried to take over the world in previous times, he constantly failed due to the intervention of Sonic, but when the hedgehog once again prevented him from taking over the world, he broke the last drop of patience and he decided to resort to the most extreme measures - to create a new world, he need to destroy the existing one, and for this, in the bowels of the Iron Canyon Zone, he creates a Nuclear Weapon and launches it into Green Hill Zone, where Sonic is now and there it has already begun to gradually work, which negatively affects the environment around Sonic. Sonic notices that something is wrong and realizes that this is the work of Eggman and he immediately begins to act. Eggman's plan is working, but it can still be prevented. To prevent it, requires six Chaos Emeralds, then reach the Reactor Core Zone and finally destroy the power source of the Nuclear Weapon. After that, the entire Christmas Island will begin to recover. If the Chaos Emeralds weren't collected, but the power source of the Nuclear Weapon was destroyed, then zones will stop being destroyed, but won't be completely restored. If the required six Chaos Emeralds will be collected and the power source of Nuclear Weapon will be destroyed, the world will be completely restored. Join Sonic to save the world from complete destruction!