The Sega CD add-on accessory for the Sega Genesis console isn’t exactly the best-remembered gaming peripheral of the 1990s. However, it had a couple significant standout games in its short lifespan, one of which celebrates its 25th anniversary this fall – Sonic CD.

Released on September 23, 1993, Sonic CD pushed the speedy platforming series to the next level in a number of ways. The game was actually originally going to be an enhanced port of Sonic the Hedgehog 2, but after Sonic 2 only saw so-so sales figures in Japan, the development team at Sega shifted to making it an entirely new experience.

The story of Sonic CD followed the titular blue hedgehog in his quest to defeat the antagonistic Dr. Robotnik – this time, via time travel. Robotnik has taken over the extraterrestrial Little Planet in order to turn it into an unstoppable base of operations; he also created a dangerous new rival for Sonic in the form of Metal Sonic. Metal Sonic kidnaps Amy Rose (who claims to be Sonic’s girlfriend, though Sonic has some qualms with that) in order to lure Sonic into chasing him throughout time.

Those two key characters to the story – Amy and Metal Sonic – were both debut characters when it came to this game (Amy had previously appeared briefly in the Sonic the Hedgehog manga, but this was her first video game appearance). Both have since gone on to become significant characters in the franchise at large, with Amy eventually becoming a regular playable character in various other main Sonic entries, and Metal Sonic alternating between a playable option and a recurring rival.

Sonic CD received critical acclaim upon its release, with particular praise going to the time travel mechanic – which allowed players to re-play the same stage in either the present, past, or future, encouraging them to replay the same stages over again in order to explore what each iteration had to offer. The soundtrack, handled by Naofumi Hataya and Masafumi Ogata, was also a standout; Hataya was a big fan of popular club musicians of the era, and the soundtrack reflects that.

The game sold more than 1.5 million copies for the Sega CD, making it the platform’s best-selling title. It would eventually be ported to other systems, both by itself and as a part of the Sonic Gems collection. The game’s story was also adapted into the Sonic the Hedgehog comic book canon as well, and multiple levels from Sonic CD were updated for Sonic Mania.