If you own Sonic the Hedgehog 4: Episode 1, you'll be able to unlock extra content in Episode 2. Owners of both get free access to "Episode Metal," which adds four new levels and a playable Metal Sonic character.

This functionality recalls the "Lock-On" technology in Sonic & Knuckles, which allowed Sega Genesis owners to connect previous Sonic games to the cartridge and replay them as Knuckles the Echidna. The difference now, of course, is that there's no cartridge to connect.