Return of the Sewer King is one of the chapter books based on the characters of Hey Arnold!. It was written by Craig Bartlett and Maggie Groening, illustrated by Tim Parsons, with cover by Tuck Tucker, and Teale Wang.
The city's going through a heat wave, exacerbated by the fact that the water keeps getting turned off every night between midnight and one, and no one can figure out why. Arnold and Gerald suspect the Sewer King is behind it, so they descend into the sewers to find him, even though the others warn them about the urban legend of alligators in the sewers (which makes Harold cry, since he had a baby alligator named Stubby as a pet who disappeared one day). Sure enough, the Sewer King is blocking the water to make a pool for his Royal Guard of alligators to swim in, and he sics the biggest gator on the boys — but Arnold recognizes it as Harold's pet Stubby, and when he calls it by its name it becomes completely tame. They then convince the Sewer King that the gators would be happier aboveground, and send them all to Harold's uncle's gator farm in Florida.