After escaping evil Baron Umbra, the sticky Underlings, and other creepy creatures of the Underwhere, Edison Beaker and his young sister, Tesla, are safe at home. And then their creature friend Knox comes calling. She needs help, back on the other side of the Night Door. Edison desperately wants to follow family tradition and become a true Creature Seeker, protecting the world from monsters and mayhem. So what else can he do but follow Knox? (With pesky Tesla and her escape-prone hamster Scuttlebutt in tow, of course!) Edison and crew stumble into a mysterious city that's sinking into darkness. Edison could be a hero and help restore light to the city and its inhabitants who are crushed by gloom. But who can he trust to help him?