Young sleuths stumble on a demonic conspiracy to blow the lid off Slick City's new sports arena. Just for a start. Goblin Fizz Marlow and troll gearhead Tank Wrenchlin are convinced that slimeball schoolmate Rizzo Rawlins' supposedly homemade battle bot illegally incorporates professional-grade code and components. Their search for proof leads them and elven trainee wizard Aleetha to several puzzles. Who is the mysterious "Codex," and why is he (or she) supplying Rizzo as well as hacking Slick City's computer systems to threaten disaster if the just-finished Slurp Stadium is opened for the upcoming Battle Bot Cup? How did the stadium come to be built over a magic stone that could, as old maps hint, serve as a portal to demon worlds? What can a trio of fourth-graders do, opposed by corrupt officials, a bully with a pair of hulking hench-ogres, and a local business tycoon withas it turns outa high-tech hand-held demon controller? Well, plenty, though not without a few missteps, help from a surprising temporary ally, and lots of climactic bot-smashing. In a slick mix of narrative blocks and panels of nonscary monsters delivering punch lines in dialogue balloons, the exploit careens along to a triumphant close. High-energy high jinks in a multicultural, or at least multispecies, setting. (Graphic/mystery/fantasy hybrid. 9-11) Copyright Kirkus Reviews, used with permission.