Murray, Mina, Connor and Shen accidentally find a strange ship that has run aground near their town. The ship is surprisingly similar to a longship - an ancient Viking ship, but with a torn sail and a name inscribed in Greek letters: "Metis". On board the ship, the guys find traces of mysterious incidents, a strange cube, all covered with numbers, and a notebook with the words written on the cover: “Ulysses Moore, notes for the thirteenth book”...Four friends decide to go to sea, and the ship will take them where their imagination and fantasy have not yet taken them.