Eggman Adventure (AKA Eggman 1, and Egg-Zotic in United Kingdom) is a SEGA game starring Eggman in his first game where he is the main character.
Part 1's Story
Shadow the Hedgehog is carried away by a dinosaur, and Dr. Eggman coincidentally is looking for him. He finds out that Shadow is missing and sets off, running into Sonic the Hedgehog, whom he tells where he is going. Sonic thinks Eggman is lying and goes to Tails, who makes inventions to stop Dr. Eggman, as the first Chaos Emerald happens to be there.
Part 2's Story
Shadow and Rogue, freed from Prehistoric Kingdom by Sonic, set out to find Eggman and see what he wants. Unfortunately, they run into Knuckles and Rogue is captured (if Shadow loses boss fight. If he wins, it will unlock Part 8, but it is impossible to win until you can play as Super Shadow) and Shadow sets off to find Rogue. He finds Tails who is creating something- which is the Electron 3.4 in Part 1- and Shadow teams up with Tails to get back to Rogue, who is secretly keeping the second Chaos Emerald.
Part 3's Story
Silver, in the future, is keeping 5 of the Chaos Emeralds but notices 2 are missing. The 5th one is stolen by Metal Sonic, and Silver sets off to find them. After fighting Anti-Blaze, it is revealed that he was making a present for Sonic to make him forget the Iblis Trigger thing. Blaze joins after curing a very sick Rogue with fire. Metal Sonic appears with all the Chaos Emeralds as Silver accidentally left them unattended.
Part 4's Story
Rogue tries to escape Knuckles' prison, but when she tries to escape, she falls on Knuckles, accidentally hugging him. She faints, and Knuckles is shocked. He goes to find somebody with a cure, and finds Blaze who betrays him to go with Silver. He goes to get revenge on Blaze, but finds Eggman on the way.
Part E's Story
Back to Eggman's story, he tries to unfreeze himself after a battle with the Freez-o-bot. He runs into Knuckles and says that Sonic is attacking. They work together to defeat the Sonic-X, which looks exactly like Sonic except with a robot voice and a few robot parts (not Metal Sonic). The two find a Chaos Emerald.
Part X-5 Story
This was taken out for unknown reasons, but it was supposed to be about Sonic-X finding all 7 Chaos Emeralds and Metal Sonic scattering them throughout the world, then revealing that Sonic-X wasn't built by Tails.
Part 5's Story
Sonic meets Sonic-X, and Sonic-X takes his voice and form. Mighty the Armadillo comes and hurts Sonic, thinking he is Sonic-X, after Tails tells him about it. Silver and Blaze then come, and Knuckles tries to punch Blaze, but Blaze accidentally shoots Knuckles with fire.
Of right now, Part 6 and 7 are unknown.
Part 8's Story
This was going to be the final boss, but was changed to a Bonus Story. In this story, Dr. Eggman creates the Paralelliverse, with many interruptions while he is making it. On accident, he creates people in that universe, who do and look exactly like them, except opposite colors. Because of the people, nobody can enter the alternate universe without being hurt, and both people would get hurt. Sonic is the final boss, and after he is defeated, it shows the case of a future game called "Sonic: Two Universes" which also reads "Cinos: Owt Sesrevinu" in the other universe.