Sonic the Hedgehog 4: Episode Metal is a free downloadable "Lock-On" part of the episodic game Sonic the Hedgehog 4, released along side of Episode II. It enables any player who owns both Episode I and Episode II on the same console to play Episode Metal, thereby acting very similar to the Lock-On Technology pioneered by Sonic and Knuckles. Episode Metal explains the backstory to Metal Sonic's return in Episode II and also serves to connect Sonic 4 to Sonic the Hedgehog CD, befitting the latter game's recent re-release on digital download services.