Sonic the Hedgehog 4: Episode Metal is an additional episode of Sonic the Hedgehog 4 in Sonic the Hedgehog 4: Episode II, which is unlockable to players who also have Sonic the Hedgehog 4: Episode I on the same device. The extra episode ties together the events of Sonic the Hedgehog CD to those of Sonic the Hedgehog 4, beginning with the formal return of Metal Sonic.
The extra episode is the result of Episode II detecting Episode I on the same system, similar to the lock-on cartridge feature of Sonic & Knuckles onto Sonic the Hedgehog 3 (Sonic the Hedgehog 3 & Knuckles), Sonic the Hedgehog 2 (Knuckles the Echidna in Sonic the Hedgehog 2) and Sonic the Hedgehog or any other Mega Drive/Genesis title (Blue Sphere).
Once Sylvania Castle Zone Act 1 is completed in Episode II, the game will check for a save file of Episode I with at least Splash Hill Zone Act 1 cleared the next time the player continues the main game. If that requirement is met, the player can access Episode Metal as an extension of the Episode II world map.
Immediately 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 Metal Sonic. 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.
Being an extension of the Sonic the Hedgehog 4 saga, the game is handled the same way as the other Episodes, with a few differences.
Metal Sonic bears nearly the exact same gameplay mechanics as Sonic does in Episode I. However, due to the additional content being built from Episode II, it shares the same modified physics engine.
The Acts are reworked versions of pre-existing Zones from Episode I, specifically all of the "Act 1" stages of the four main Zones. The zone order is in reverse compared to Episode I, with the first stage being Mad Gear Zone to the final one being Splash Hill Zone. The level designs are also more difficult than usual, having additional enemies, bottomless pits, and spikes.
Interestingly, when Metal Sonic clears an Act and dashes past a Signpost, it spins and changes from displaying an image of Sonic to an image of Metal Sonic. This mimics the same action that occurs when Sonic clears an Act and zooms by a Goal Plate, which normally changes the image of Eggman to one of Sonic. Additionally, the picture of the Power Sneakers Item Box, which usually shows Sonic's shoes, now shows Metal Sonic's foot.
- Act 1 - Metal Sonic Returns!
- Act 2 - The Altar's Hidden Treasure
- Act 3 - Electric Road
- Act 4 - In Pursuit of Sonic...
- Stardust Speedway (cutscene only)
- Mad Gear Zone - Act 1
- Lost Labyrinth Zone - Act 2
- Casino Street Zone - Act 3
- Splash Hill Zone - Act 4
- Stardust Speedway, as it is depicted in Episode Metal, is the same as it was in Sonic Generations.
- The introductory cutscene begins with the Stardust Speedway Zone 3 title card from Sonic CD, the same Round and Zone where the Metal Sonic race took place at.
- As Metal Sonic reactivates, the music turns into an instrumental version of "Sonic Boom", which is the main theme of the North American version of Sonic the Hedgehog CD. When Metal departs Little Planet and flies towards Mad Gear Zone in Sonic's world, a rendition of "You Can Do Anything" plays, being the same game's main theme in the Japanese and European versions.
- The physics of Episode II are used in the game, but the gameplay elements are from Episode I.
- A beta of the Windows version of Episode II was leaked on Steam in April, though it was quickly removed. As a result, those who owned Episode I had access to Episode Metal as well.
- All of Metal Sonic's sound effects are taken from his added playable appearance in Sonic Adventure DX: Director's Cut.
- The sound that Black Doom made when he appeared to Shadow in Shadow the Hedgehog was reused when Metal Sonic obtained the energy in Lost Labyrinth Zone.
- The introductory cutscene set in Stardust Speedway may be mistakenly absent in the mobile version of the game depending on the user's device.