Concept art for Sonic 3 might have just revealed how Metal Sonic and Amy join the story in Sonic 3 - and it could centre around time travel.

In Sonic 3 ’s mid-credits stinger, an army of Metal Sonics show up to neutralize Sonic. But a savior arrives: Amy Rose, cloaked and armed with a giant mallet, and she lays waste to the robotic Sonic clones. Neither Metal Sonic nor Amy speaks during the scene, which leaves their places in the story open-ended for now.

In addition to Shadow's introduction in Sonic the Hedgehog 3, the film adds another familiar face during its mid-credits scene.