One series, Sonic the Hedgehog (often refered to as SatAM), was made for Saturday morning television and was a serial based show featuring more characters and darker storylines. It had 26 episodes and ran from 1993-1994.
The other, called The Adventures of Sonic the Hedgehog was made for weekday syndication and only featured Sonic and Tails in more humourous and slapstick situations. The Adventures of Sonic the Hedgehog ran from September to December 1993 and had a total of 65 episodes.
While the show was popular, the real fans favoured the Saturday morning show. But for some reason, a few years after both series had come to an end, DiC decided to produce one more episode based on the Adventures series! This is probably due to the fact that it deals with Christmas, something that just wouldn't feel right on the SatAM series.
Enough about the history! Let's talk about the episode: Santa is making a public television appearance to announce his retirement. But instead of cancelling Christmas, he names his replacement - Robotnik Claus! It's up to Sonic and Tails to stop Robotnik Claus from taking presents instead of giving them in order to return order to Christmas in Robotropolis. SPOILER: The show ends with Sonic freeing a captive Santa and Santa teaching Sonic how to obtain Ultimate Velocity (Super Speed) in order to save Christmas. Santa is so impressed with Sonic's work that he retires for real and hand the duty over to Sonic - who accepts! What?!
It is nice to get most of the returning cast, including Jaleel White (Steve Urkel) as Sonic, Long John Baldry as Dr. Robotnik, Gary Chalk as Grounder, and Phil Hayes as Scratch. The only cast member to not return is Christopher Evan Welch who was eleven when he voiced Tails but hit puberty and thus had a voice that was too low for Tails in this special.
All in all, this is a goofy special that fits very well within the context of the original show but won't become a perennial favourite. You can find it on DVD along with three other episodes from Sonic Underground.