About 4000 years ago, a sentient weapon was created by an ancient civilization. After being lost for millennia, the weapon, called a Gizoid, was unearthed and researched by Prof. Gerald Robotnik. It remained dormant for another half century before being discovered by, of all people, resident would-be ruler of the world and Gerald's grandson, Dr. Eggman. Frustrated at his inability to get the dormant Gizoid to work properly, Eggman dumped the contraption at Emerald Beach... where it was discovered once again, this time by planet's resident hero, Sonic the Hedgehog.

The Gizoid, which Sonic dubs Emerl, gets wrapped up in the affairs of Sonic's friends, allies and rivals. Through his encounters with Sonic, Tails, Knuckles, Amy, Cream, Rouge, and Shadow, Emerl grows and evolves from a soulless robot into a fun-loving mech with personality traits from all of the above. But while the gang is having fun raising their new mechanical toy, there are those who recognize Emerl's true power. Now that the Gizoid is working again, Eggman's got his eye on it, Rouge wants to turn it into a master thief, and Shadow senses that, despite their efforts to humanize him, Emerl is still designed and programmed to be a weapon of mass destruction...