Sem surfaces inside a magnificent lost city illuminated by crystal stars in its stone ceiling.
The episode is a narrative story told in French, following the life of Sem, a member of a nomadic desert tribe called the people of the dunes. Sem hears tales from traders about distant cities with white houses and towers, and legends of a forgotten empire hidden beneath the desert. He becomes obsessed with finding the hidden entrances to this underground kingdom. One evening, while bathing in a lake, Sem slips on a stone and discovers a staircase underwater, which he follows down into the dark. He emerges into a vast, abandoned city made of white stone, illuminated by torches and a ceiling with glowing crystal stars that mimic constellations. Sem explores the city's streets, temples, and statues, marveling at its beauty and wondering about its lost inhabitants. He returns to the surface but keeps his discovery secret. Over the years, Sem finds other hidden doors to the city, often through dreams or natural landmarks like twisted trees. He marries, has a daughter named Feni, and becomes chief of his tribe, visiting the city less frequently. One night, while out with Feni, they are attacked by horse-riding pillagers who want to enslave them. Sem recognizes a nearby twisted tree as a hidden door and escapes with Feni into the underground city. There, he finally reveals the secret to Feni, showing her the city and teaching her to recognize the doors. They explore together, imagining stories about the city's past. After Sem's death, Feni continues to discover new doors and eventually passes the secret to one of her own children. The story concludes by noting that the secret of the city under the sands is always known by someone in the desert, preserved through generations.
