Herman Munster [Fred Gwynne] and Lily Munster [Yvonne De Carlo], following a will from Cavanaugh Munster, discover they have inherited Munster Hall, an English manor in Shroudshire, and that Herman has inherited the designation of Lord of the Manor, earning him the title of "Lord Munster." The couple sails to England aboard the famed American transatlantic liner SS United States to claim their new life. On the voyage, Herman battles seasickness, while Marilyn [Debbie Watson] meets a new love interest, Roger Moresby [Robert Pine]. Meanwhile, Grandpa [Al Lewis] experiences a surprising mishap when he accidentally swallows a wolf pill and is transformed into a grey wolf, forcing the family to navigate British immigration and customs with some cunning and care.
Cousins Grace [Jeanne Arnold] and Freddie [Terry-Thomas] react with fury and jealousy, furious that the American Munsters are seizing the manor and that Herman will wear the title of Lord Munster instead of Freddie. They are aided by their mother, Lady Effigy Munster [Hermione Gingold], who hopes to secure the estate for herself and her family. Freddie even disguises himself as a ghost in a bid to scare off the newcomers, but a wary encounter with Herman sends him fleeing. Driven by this dispute, Grandpa secretly investigates Munster Hall to uncover the truth behind the mansion’s aura, and he uncovers a secret: a counterfeiting operation hidden in the basement, run by a mastermind known as the Gryphon.
Later, a high-stakes race becomes the focal point of the conflict, with Herman taking the wheel of Grandpa’s custom dragster, the DRAG-U-LA. Grace and Freddie hatching a plot to eliminate Herman before he can win, aided by the Gryphon who pretends to be Roger, though Lily is there to help thwart the scheme. In the end, Herman’s perseverance pays off, and he crosses the finish line first, securing the victory against the schemes of his English relatives.
The ruse and the counterfeit scheme are exposed to everyone, including the ship’s crew and the local authorities. The ship’s butler, John Carradine as Cruikshank, and his daughter Millie Maria Lennard the barmaid, are revealed to be accomplices who helped the Gryphon advance his plan to become Lady Munster. They are all apprehended by the police, and the confrontation leads to justice being served. In a dramatic moment, Herman captures Freddie and Grace by throwing tires over them, ending their threats. Lily and Eddie [Butch Patrick] step in to help, while Lady Effigy [Hermione Gingold] is escorted to the Shroudshire police station with her loyal butler. With the antagonists dealt with, Herman and his family decide to donate Munster Hall and the surrounding land to the city for historic preservation. Roger and Marilyn’s budding romance brings a touch of warmth to the journey, and, with their new connections settled, the Munsters return to their American home, having secured a legacy that blends their quirky, modern family with a sense of grown-up responsibility.
