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.

Following his crushing and brutal defeat by Sonic on Stardust Speedway, a severely damaged and powerless Metal Sonic falls as he is left behind on Little Planet. A long time afterward, a screen floats down to the robot's remains. The screen shows Dr. Eggman as he finds the ruined Metal Sonic, who he shocks and reactivates. Eggman then commands the crippled Metal Sonic to leave Little Planet and summons him to Mad Gear Zone, which was built for the specific purpose of repairing and enhancing the robot. Revitalized, Metal Sonic is sent off to eliminate Sonic once and for all. He soon arrives at the Lost Labyrinth Zone and explores the deeper parts of the ruins. Once he reaches an altar in a secret room hidden within the darkness, he finds a Mysterious Orb comprised of powerful energy, which is exactly what Eggman was searching the Labyrinth for. Metal Sonic approaches the orb and absorbs it into his system. With a massive surge of power, his body erupts with purple sparks and a purple aura as he tears through the stone walls of the Labyrinth with ease, leaving for the electric roads of Casino Street Zone. In pursuit of Sonic, Metal Sonic finally reaches Splash Hill Zone. However, while he manages to locate his organic counterpart, he reaches the end of the Zone only to find Sonic and Tails taking off on the Tornado, heading for Sylvania Castle Zone. Metal Sonic quickly notices a rocket with Tails' insignia placed not too far from an open Capsule. Metal Sonic unleashes his new special attack to scare the surrounding Animals away as he takes the rocket for himself, flying off to trail the brotherly duo.