Imagination Invaders is a 3D adventure/action game based on the popular Cartoon Network cartoon. Players control Bloo in a 3D world as he tries to help his friends and rescue Madame Foster. Players can jump, shoot spitwads, and search objects in the world to aid them in their quest. Using the touch screen they can also call in Helper characters. Each level has an objective which is usually helping one of the other Imaginary Friends, capped off with a boss battle at the end. All of the objects in the world are interactive in some way; either searchable or play a "funmation" for Bloo. Players also have to conquer some light platforming and enemy avoidance. In between levels, touch screen mini-games are used.
The overall plot was a sequel to the episode Make Believe It or Not, and was written unique to the game. Minor voice acting by the two main characters of the show were put into the audio. Many of the Foster's characters make cameos throughout the game.