The Game Boy Advance adaption follows the television show's story closely, involving the characters who are from a video game appearing on Earth, and Ace ventures into the Carnival of Doom to find the pieces of the Amulet and defeat Lord Fear, eventually sending the villains back to the Sixth Dimension, the homeworld of the characters.
The gameplay of the Gameboy version is a simple side scroller platform game where a player controls Ace and moves him around the carnival's attractions. At the end of the each level is a piece of the Amulet and a boss battle against an antagonist suited to the environment. The levels are based on attractions that are usually seen in an amusement park of small funfair (e.g. a ghost train, carousel, rollercoaster, etc.), the final level being the Haunted House where Lord Fear is battled. The player can be ambushed by the enemies whilst navigating their way around the carnival, leading to miniature boss battles. Two minigames are unlocked throughout the course of the game, one being a shooting gallery and the other a strength-testing game, where the player has to time Ace when he hits the machine. If he fails, Ace will toss the mallet used into the air, but it will fall down and knock him out.
Source: Wikipedia, "Ace Lightning (video game)", available under the CC-BY-SA License.