In the 21st century, a blast of cosmic radiation bombarded Terra-12, a deep-space outpost of Earth, hideously mutating all transplanted life. A fleet of savage beings followed the radiation wave and invaded the planet, and began the systematic destruction of all remaining sentient life. Years of battling the alien 'governors' have gone by, and now only one hope survives to avenge the desperate terran colonists.
The player controls the titular character through six stages of increasing difficulty, facing an alien "governor" boss (which are so large as to be considered levels in and of themselves, as some of the bosses take up several screens) at the end of each level, followed by a "duel" level against other Robo-Kid sized robots. Many levels branch into others, giving the player the choice over which zone to enter next, increasing playability.
Robo-Kid can collect four different weapons (whichever weapon is selected is lost when Robo-kid loses a life) in addition to his default gun, collect powerups for a shield that activates on enemy contact, plus rapid fire and speed powerups. The player can also encounter a friendly dinosaur-looking robot that sells weapons and shields to Robo-kid using extra lives as currency.
SEGA Mega drive/Genesis
Source: Wikipedia, "Atomic Robo-Kid," available under the CC-BY-SA License.