Séfar (in Arabic: سيفار) is an ancient city in the heart of the Tassili n'Ajjer mountain range in Algeria, more than 2,400 km south of Algiers and very close to the Libyan border. Séfar is the largest troglodyte city in the world, with several thousand fossilized houses. Very few travelers go there given its geographical remoteness and especially because of the difficulties of access to the site. The site is full of several paintings, some of which date back more than 12,000 years, mostly depicting animals and scenes of hunting or daily life which testify that this hostile place has not always been an inhabited desert. Local superstition suggests that the site is inhabited by djins, no doubt in connection with the strange paintings found on the site.

The story is about a young boy named Lubicchi and a trashman named Poupelle who meet on a Halloween night in a city that is full of tall chimneys. The chimneys pour out black smoke and the true sky cannot be seen.

Children are lured to Metropolis only to find that the renowned, mythical city may not be the happy-go-lucky utopia they'd thought it would be.