A real family and a home in the ice city beneath the sea – Mats should be truly happy. But here, no one really understands him, not even his own twin sister Finja. Right now, she has nothing on her mind but her training to become a Source Guardian, practicing ice magic instead of taking care of her brother. But just while the two are arguing, a much greater disaster is brewing above them: the gods awaken the water serpent Bölvun. Suddenly, everything is at stake. Mats and Finja have little time left to save themselves and to protect Rulantica from destruction.