Murray, Mina, and young Captain Connor embark on their third voyage across the Imaginary Seas.

This time they find waiting for them at Kilmore Cove the worst pirates of the seven seas, and a Long John Silver increasingly determined to fight the Imaginary Seas Company and its mad superintendent Larry Huxley.

First, however, they must find out where Ulysses Moore is hiding and free the other rebels, and the only way they can do that is to tie themselves up with ropes and carabiners and start descending to the center of the Earth ...