At Marilyn's birthday party, Eddie gives her a ring he found in the attic. Grandpa claims it is the Fregosi Emerald, which causes bad luck.
After wearing it, Marilyn comes home from a date, distraught. "I never thought I'd get home alive!" Herman does not believe that it is unlucky, so he has Lily wear it. She experiences bad luck, too. The breakfast she made becomes extremely heavy, causing the sides of the pot come off, while the bottom remained on the stove.
Herman then decides to put on the ring, but after bad luck, he cannot take it off. Nothing can make it come off--even the soap exploded. Then Grandpa has an idea--they cannot get the ring off Herman, but maybe they can get the curse off the ring.
Grandpa calls Transylvania, asking for a member of the Fregosi family. He learns that there is only one left--Henry J. Fregosi, the head of Amalgamated Motors in Detroit.
The Munsters visit Detroit to see Mr. Fregosi. He denies that he has a magical family, until they show him the ring, which frightens him. He then takes them into a hidden room where he performs his black magic. He lifts the curse off the ring (after admitting the magical tactics he uses to eliminate competition), and the Munsters go home.
Marilyn is thrilled with the ring. Eddie comes down, having found a ruby on a chain. Grandpa recognizes it as being much worse than the Fregosi emerald, and hurls it out the door--and accidentally into Herman's mouth.