|The cover art|
After avoiding Doctor Robotnik's robots and death traps (Robotnik being the main villain of the game), Sonic meets up with several rebels. The rebels say they should find Doctor Kintobor to help them. Sonic revealed Kintobor is Robotnik. Sonic tells them the origin of Robotnik.
Sonic was a brown hedgehog that befriended the benevolent Doctor Kintobor. Sonic ran on one of Kintobor's treadmills, but ran too fast. This caused an explosion that (due to "the cobalt effect") turned him blue. Kintobor attempted to use the Chaos Emeralds (which he harnessed by the "Retro Orbital Chaos Compressor") to remove all evil from their world. However, Sonic spilled a soda and egg on the machine turn Kintobor into the evil, egg-shaped Robotnik.
After explaining this, Sonic leaves to fight Robotnik. He reenacts the event of his game. In the end, Robotnik escapes via his hovercraft.
As mentioned before, the comic was supposed to explain the plot of Sonic. However, the comic (even at the time) isn't an "official" part of the Sonic mythos. As such, the Japanese explanation is treated as the "true" origin as oppose to this.
http://info.sonicretro.org/Sonic_the_Hedgehog_(promotional_comic) (which has a summary of the plot and scans of the comic)