Far from the Terran colony's Homeport on the planet Astra, young Dalgard Nordis and his merman companion Sssuri are suddenly confronted by their old enemies, the alien Astrans. Within the ruins of the Astrans' former citadel the two discover that remnants of this nonhuman race, which had once ruled the entire planet, are struggling, to recover their lost knowledge and thus regain their power. Dalgard realizes that the safety of the Terrans is seriously threatened by this, and there is no hope of warning his people in time. When a space ship arrives from Terra, its crew ignorant of the existence of a Terran colony on the western continent across the sea, the aliens enlist the spacemen's aid. Of the members of the crew only young Raf Kurbi instinctively mistrusts the Astrans. Through a series of weird and exciting adventures among the ruins and in ancient underground tunnels, Raf eventually meets Dalgard and joins him in the fight against the aliens.