As the assistant to Margo Maloo, Charles has gone on assignment in some pretty interesting locations. He's been to a vampire's lair, an ogre's den, and even a monster grocery store! But he's never been to Downside, Echo City's underground monster metropolis that is super mysterious and definitely off limits to kids. But a settlement of giant spiders is about to be dynamited by unwitting construction workers - and, coincidentally, one of those giant spiders "accidentally" bit Charles. His cure and the spiders' salvation lie in the Downside - if Charles and Margo can make it in time.