Petrian used to be a normal modern world when suddenly gates connecting to a dungeon that was filled with monsters appeared. After that, humans discover that they could capture the monsters and use them as pets to defeat other monsters. Since then, Petrian has become a world of pet tamer. A thousand years later, a Grade E city was ravaged by a strong monster and many people died. One of them was Lucas’s father. Not only that, Lucas was heavily injured but he miraculously woke up from a coma. When he woke up, he found out that he owned the Pet Nursery System. Lucas used the system and wanted to open the best pet nursery in the universe.