Here Be Monsters!
 Arthur lives with his adoptive grandfather, William, in the complex network of tunnels beneath Ratbridge. William went underground after being unjustly accused of attempted murder. Arthur emerges at sundown in search of food, aided by a pair of hand-cranked mechanical wings. He also carries a doll—an effigy of his grandfather with his wings—which serves as a walkie-talkie, allowing him to communicate with his grandfather. On one such expedition Arthur witnesses an illegal cheese hunt. He follows the hunters and the captured cheeses back to the Cheese Hall. Arthur's wings are stolen, and he is almost captured, by Archibald Snatcher, the leader of the once-powerful Cheese Guild. Arthur is rescued by Fish, a boxtroll, who takes Arthur to meet his friend, Willbury Nibble. Willbury is the proprietor of a former pet shop called Here Be Monsters. He shares his home with several boxtrolls (Shoe, Egg, and Fish) and a cabbagehead (Titus). Such creatures usually live underground and are collectively termed Underlings. Arthur's new friends intend to use their knowledge of the Underground to help him return to his grandfather, but they quickly discover that all of the entrances to the tunnels have been sealed. Back at the shop, a mysterious individual sells Willbury a number of miniaturized creatures and attempts (unsuccessfully) to buy their full-sized counterparts. Later, during a shopping trip, Willbury and Arthur are surprised to find that miniaturized Underlings are being marketed to Ratbridge's women as the latest fashion trend. They also seek out Willbury's friend, Marjorie. An inventor, Marjorie is camped out at the patent office, waiting for the return of the clerk who left with the prototype of her latest invention. When they return to the shop, the place is a shambles and the Underlings are missing. While assessing the wreckage they are visited by Kipper and Tom, members of the crew of the Ratbridge Nautical Laundry, soliciting customers. The Laundry is headquartered on a ship that became stuck in a canal and is staffed by a combination of men, rats, and crows. (Playing against stereotype, the rats are portrayed as intelligent, congenial characters who share leadership duties with the men. It is alluded to that the crew were pirates before going into business as a laundry.) Several of their crew have also gone missing and they are happy to join the search for the Underlings. The group begins surveillance of the Cheese Hall. After seeing the Guild members leave on another illegal hunt, Arthur slips inside. He retrieves his wings and helps a number of Underlings (including his friends) escape from the dungeon. They return to the Nautical Laundry. However, corrupt police officers arrest Arthur and hand him over the Guild; he is imprisoned in the dungeon. The prisoner in the adjoining cell—Herbert, the Man in the Iron Socks—tells Arthur that the Guild is creating miniature creatures, although he doesn't know why. He helps Arthur retrieve the keys to their cells from the sleeping guard. Meanwhile, Willbury, Marjorie, and several members of the Laundry disguise themselves as boxtrolls and enter the Underground via an open tunnel outside the city walls. They locate Arthur's grandfather, but soon the party is caught in the traps used by the Guild to capture Underlings and taken to the laboratory. Marjorie recognizes an enlarged version of her stolen invention—now revealed to be a size transference device. A gigantic rat emerges from a pit in the middle of the floor. The Laundry members recognize it as one of their missing comrades, although greatly enlarged. Snatcher reveals that the Guild has been transferring the size of captured Underlings to "the Great One", and feeding him with the captured cheeses, so that they can wreak vengeance on Ratbridge. Still believing her to be a boxtroll, the Guild transfers Marjorie's size to the rat, leaving her only seven inches tall. Arthur is then brought into the lab and is to be the next shrunk. However his guard is actually Herbert, disguised as a Guild member. Herbert is able to free the captives using his "walloper"—a large mallet—and knocks a hole in the wall to allow their escape. Arthur again retrieves his wings—and Marjorie's prototype—and the group quickly returns to the ship. The Guild proceeds to dress the Great One in battle armor, and the hole in the wall only allows him an easier exit. Seeing the danger the town is in, the heroes return to the Cheese Hall hoping to stop the Great One. They activate a large electromagnet, which draws the iron armor back toward the hall. However, the pressure within the armor is so great that the Great One explodes, covering the town in a layer of cheese. The shock wave triggers a collapse of the underground tunnels below the hall. Using the prototype size transference device, the miniaturized creatures are returned to their normal size. William is able to clear his name, and he, Arthur, and Herbert take up residence in comfortable rooms above the old pet shop.