Sonic the Hedgehog CD, commonly called Sonic CD, is a 1993 side-scrolling platform game published by Sega for the Sega CD. The story follows Sonic the Hedgehog as he attempts to save an extraterrestrial body, Little Planet, from Doctor Robotnik. As a Sonic the Hedgehog series platformer, Sonic runs and jumps through several themed levels while collecting rings and defeating robots. Sonic CD is distinguished from other Sonic games by its time travel feature, a key aspect to the story and gameplay. By traveling through time, players can access different versions of stages featuring alternate layouts, music, and graphics based on the time period.