After a difficult test, the lazy human youth Jonathan (voiced by Andy Samberg) finally walked into the palace of marriage with the vampire girl Mavis (voiced by Selena Gomez), and the monsters represented by Mavis's father Dracula (voiced by Adam Sandler) and the relationship between humans has never been closer. Soon after the marriage, Mavis gave birth to a lovely son Dennis. The little guy grew up day by day, and his grandfather Dracula loved him very much. He eagerly hoped that Dennis could grow up to be an excellent vampire. In contrast, Mavis believed that her son was an ordinary human child. She hoped to take Dennis into civilized society and stay away from the violent and bloody monster world. In order to prevent his daughter and her family from leaving, Dracula tried to get Mavis away, and at the same time called Frankenstein, Wolf Man, Invisible Man, Mummy and other buddies to launch a vampire training for Dennis in a limited time. The differences in ideas between the two generations are no exception in the world of monsters...