A ship carrying the Spanish Ambassador Senior Sel Fuego and his mysterious cargo runs aground in the Sargasso Sea, and Fuego seems to disappear into a mirror. While Verne is plagued by strange visions, the Foggs and Passepartout go to investigate, and the Foggs pass through a dimensional curtain. They are imprisoned, and find out that Count Gregory and the League of Darkness are behind the sceheme. By entering the other dimension, Phileas and Rebecca caused the creation of "Similars" back in the real world. Working together with the "Similar" of Fuego (who is also a prisoner), the three duplicates plan to kill Queen Victoria. Verne realizes his "friends" are imposters and convinces Sir Chatsworth. The two of them fail to convince the Queen, however. The real Phileas asks that Rebecca kill him, as this will destroy his own Similar. Apparently the Holy Grail, Fuego's cargo, recognizes his willingness to sacrifice himself and frees them. With the aid of Passepartout, who has been trapped aboard the Aurora within the magnetic field, they return to the real world, which destroys their Similars.

When Jules contracts yellow fever in Tangiers, a famous doctor takes him for treatment to a Corsican hospital that is secretly controlled by the League of Darkness.