Was actually two separate mini-series episodes edited into a single movie for its American release.
The movie is based on game characters and setting styles set before the character redux introduced in Sonic Adventure (1998). These styles include Sonic the Hedgehog, 2, CD, 3, and Knuckles. Sonic CD is particularly relevant, considering the appearance of Metal Sonic and certain dynamic running animations of Sonic resembling the introduction of the same game.
The American version was released in 1999, the same year "Sonic Underground" was first broadcast.
More episodes may have been planned.