Thorgal, his family and their friends Darek and Lehla have left their island to sail south and look for a safer place to live. After a storm scatters them, Thorgal and WolfCub's boat flounders on reefs at the foot of a strange, mist-covered cliff. One perilous climb later, the little girl is trapped by a mysterious, invisible force, and her father, after going for help, discovers a very curious city cut off from the world...