Some time after the first film, Mavis (Selena Gomez) and her fiancé Johnny (Andy Samberg) are married, with the approval of her father Dracula (Adam Sandler), and the world becomes aware of (and unfazed by) the existence of monsters. Mavis later reveals to Drac that she is pregnant and a year later, she gives birth to a boy named Dennis (Asher Blinkoff), who befriends Wayne’s daughter Winnie (Sadie Sandler). Nearing his fifth birthday, Dennis has yet to grow his fangs and Drac worries that his grandson might not gain vampire powers. Noticing the dangers of Transylvania, Mavis starts to consider raising Dennis where Johnny grew up, much to Drac’s disapproval.
Drac tells Johnny (who doesn’t want to leave the hotel either) to bring Mavis to California to visit the in-laws, Mike (Nick Offerman) and Linda (Megan Mullally), but to make sure to keep her distracted so that she will not move, leaving Drac to babysit Dennis. Drac enlists his friends, Frank (Kevin James), Wayne the Werewolf (Steve Buscemi), Griffin the Invisible Man (David Spade), Murray the Mummy (Keegan-Michael Key) and Blobby the Blob (Jonny Solomon) to help train Dennis to become a monster, to no avail. Drac takes Dennis to a summer camp, where he learned to hone his vampire abilities, and discovers that the camp is safer than it was when he went there. Out of stupidity, Drac unknowingly hurls Dennis from a tall, unstable tower to pressure his transformation into a bat, but rescues him at the last second. The stunt is filmed by the campers and uploaded to the internet, which eventually reaches Mavis and Johnny who decide to hurry back to Transylvania. Without any flights available, Mavis flies straight to Translyvania as a bat carrying Johnny. Drac’s pack reaches the hotel a couple of seconds after Mavis, upon which she confronts her father for putting Dennis in grave danger and his inability to accept that he is human, promising to move out of the hotel after Dennis’ fifth birthday the following Wednesday.
Mavis invites Vlad (Mel Brooks), her grandfather and Drac’s father, to Dennis’ birthday party. As Vlad is much worse than he was when it comes to humans, Drac tells Johnny to have the human partygoers disguise themselves as monsters. Vlad receives the invitation and arrives with his monstrous bat-like servant Bela (Rob Riggle) to meet his great-grandson for the first time. Meeting him, he believes that fear will cause Dennis’ fangs to sprout and possesses a stage performer dressed as Dennis’s favorite television monster, “Kakie the Cake Monster,” to scare Dennis, but Drac shields his grandson at the last moment and exposes the deception to Vlad, who is outraged that Drac has accepted humans as guests in his hotel. Drac confronts his father about how humans are different now.
Mavis becomes upset with her grandfather’s behavior and while the family quarrels, Dennis sadly flees the hotel (followed by Bela) and enters the forest with Winnie in tow, hiding in her treehouse, but they are attacked by Bela. When Bela injures Winnie, Dennis’ anger causes him to instantly grow his fangs and his vampire abilities manifest. He begins to fight Bela, who calls his giant-bat minions. Drac, Mavis, Dennis, the rest of the monsters and (some of) Johnny’s family team up to defeat his minions. A livid Bela then attempts to kill Johnny with a stake, but Vlad appears with a change of heart where he shrinks his former servant to a harmless size and lets the Werewolf Kids lick him nonstop.
Now that Dennis has vampire abilities, Mavis continues to raise him in Transylvania and the party resumes.