The deadly worlds of the Cryptus system, constantly tormented by the gravitational and radioactive influences of their double stars, find themselves faced with a new threat - the xenos power of the Tyranids. The tentacle of Hive Fleet Leviathan reached the shield worlds of Cryptus. The Imperium gathers its forces, for the aliens must be stopped here, because next in the path of the Hive Fleet will be the home world of the Blood Angels, Baal itself. While on the world of Lysios the valiant Adepta Sororitas defend the planet from the beasts of Leviathan, a platoon of the Imperium's elite stormtroopers, the Tempestus Scions, are assigned to the service of the mysterious Inquisitor Ulrich on a dangerous and vital mission: the capture of a unique Tyranid bioform within the treacherous tidal shaft of Lysios. This must be done even if Tempestor Chavis and his people have to give their lives.