Sonic & Knuckles

Sonic & Knuckles is the fourth entry in its series, directly continuing the narrative from Sonic the Hedgehog 3. The story involves Sonic's pursuit of the Master Emerald to prevent Dr. Robotnik from powering the Death Egg, while Knuckles the Echidna simultaneously seeks revenge after a robotic attack. Players can choose between these two characters, each with distinct movement capabilities that alter level navigation.

Core gameplay retains the series' emphasis on speed and ring collection for protection. The title introduces three distinct bonus stages. Two are accessed via checkpoint stars, offering a casino themed ring gambling opportunity and a challenging orb jumping test for power ups. The third is the Special Stage from Sonic 3, where collecting blue spheres yields Chaos Emeralds.

A defining hardware feature was the cartridge's "Lock On" slot. Connecting Sonic 3 merges both games into one extended adventure with save functionality. Using Sonic 2 enables Knuckles as a playable character within that game's levels. Attaching the original Sonic the Hedgehog cart unlocks endless variations of the Blue Sphere bonus stage. These interactive features were preserved in later re releases.