The Mansion of Secrets, Camille Brissot

Burglaries are on the rise in the small town where Armand and his friend Pepper live. Pepper is determined to stop them, especially since Armand has just discovered an extraordinary gift: he can transform into a spider at will and sneak anywhere! When the children realize that the thieves are holed up in Mrs. Mesmer's mansion, a horrible old woman, they devise a devious plan to neutralize them. They sneak through secret underground passages to the mansion.