It's a special day for Mavis ([Selena Gomez](./actor/selena-gomez)) and Johnny ([Andy Samberg](./actor/andy-samberg)) as they celebrate their wedding. However, Dracula ([Adam Sandler](./actor/adam-sandler)) is filled with anxiety over the possibility of Mavis relocating out of the hotel. His resolve is tested when, a month later, Johnny shares that Mavis desires to go flying with him. During a game of hide-and-seek, Mavis admits she cannot fly for long, revealing she is pregnant. Dracula is thrilled at the prospect of being a grandfather but feels apprehensive when Mavis mentions their child could potentially be human.
As the time draws closer to the delivery, Dracula disguises himself as a nurse to sneak into the delivery room, promising Mavis that no harm will come to his grandson. On Dennis' (Asher Blinkoff) first birthday, a party ensues with Johnny's parents, Mike (Nick Offerman) and Linda (Megan Mullally), along with friends like Frankenstein (Kevin James](./actor/kevin-james)), Eunice ([Fran Drescher), Wayne (Steve Buscemi](./actor/steve-buscemi)), Wanda (Molly Shannon), Griffin (David Spade), and Murray (Keegan-Michael Key). Dracula harbors hopes that Dennis will become a vampire, even when Linda points out the absence of fangs. Mike tells Johnny he should seek employment, but Dracula reassures him with the comforting notion he already has a job at the hotel.
As the months fly by, Mavis shares with Johnny and Dracula that Dennis has spoken his first word: “Bleh bleh bleh.” Dracula conducts a check for fangs, but nothing is there. Attempting to teach Dennis how to transform into a bat when he is four, Dracula is met with only flapping arms rather than any real transformation. Though Mavis initially resists it, she reluctantly takes Dennis to Wayne's pups' birthday party. Amid playful chaos, Mavis is delighted but contributes to the mess when Dennis accidentally loses a tooth, igniting Dracula's excitement at the thought of fangs appearing.
Feeling disheartened upon hearing that Mavis and Johnny are contemplating a move to California for Dennis' upbringing, Dracula devises a plan to showcase the monster lifestyle to encourage Dennis to embrace his vampire heritage. He desires to host a series of experiences filled with monster antics to sway Dennis' decision before his fifth birthday.
When Mavis and Johnny set off to Santa Cruz for some time away, Dracula takes Dennis to the Vampire Summer Camp where he has his roots. He finds himself frustrated with the lack of fear in the monsters’ attempts to impress Dennis. Back in California, Mavis finds Johnny's family's guest room equipped with a coffin, further straining her desire to raise Dennis in a peculiar environment.
Dracula opts for a disastrous stunt, thinking Dennis will learn to fly by being thrown off a tower, but he must dive down last minute to save Dennis. Amid a chaotic series of mishaps and misunderstandings, Mavis discovers her son is missing, leading to a frenzy that results in Dennis unlocking his powers when confronted with danger.
In a climactic showdown, the family battles against the demons that threaten their safety. Unexpectedly, Vlad (Mel Brooks), Dracula's father, steps in to change the course of the altercation, ensuring the safety of his newfound family members. Ultimately, Mavis resolves to raise Dennis within the hotel walls, and as they gather back for the birthday celebrations, the night culminates in joy and togetherness, illustrating the blend of both human and monster worlds.
