The fourth game in the Scribblenauts series follows the same basic mechanics as the previous three. You play as Max, who owns a magic notebook. Scribe any word in it, and it becomes real (of course, only if the word you wrote describes a particular object). The game will understand your written words and create interactive in-game objects. They are used to solve puzzles, which earns you Starites, the ultimate goal of the game. The ability to summon any imaginable item gives you an almost unlimited number of ways you can solve any puzzle.
Scribblenauts Unlimited features a way more detailed plot than its predecessors. There's an extensive backstory that explains the protagonist's motivation in gathering Starites. We learn that Max and his sister Lily played pranks on people using their magic notebook until it got them into trouble. One day they unknowingly played a trick against an old wizard. He grew angry and magically turned Lily into stone. Max can only save his sister by gathering enough Starites. Unlike previous parts, this game is set in an open, albeit cartoonish, world, that gradually opens to you as you gather more Starites. The tasks are now assigned by the NPCs, and Starites are given as a reward for help. Thus, they serve as the proof that Max has corrected his behavior.