Scrooge, Huey, Dewey, Louie, Mrs. Beakley, Webby, and Duckworth have arrived at an old castle, which Scrooge wants to turn into a hotel. Duckworth says that the castle might be haunted, and reveals that it was once owned by a mad scientist by the name of Ludwing von Strangeduck, whose ghost supposedly still haunts the place.
Scrooge could care less about some silly ghost story, and gets everyone to help clean up the castle. Soon Mrs. Beakley and Webby have landed the roles of maids, while the nephews are serving as bellhops. Scrooge teaches the boys the method of asking for tips, and eventually gives in by tipping them, though he manages to swindle his money back from them by asking for a management cut.
After a week has passed, Duckworth seems pretty confident that there is no ghost, when the first guest, arrives. She is the Duchess of Swansylvania, a well-off woman with an eastern European accent. Soon the second guest arrives; an anthropomorphic Italian pig named Benzino Gasolini, who’s a professional playboy and race car driver seen earlier in the series.
When the third guest arrives, everyone is shocked to see that it’s actually an invisible ghost! When the ghost signs in as Dr. Ludwing von Strangeduck, Scrooge and the boys get scared and run off. The boys hide in a storage closet, but are scared off again by the ghost. The ghost later frightens Webby and Mrs. Beakley while they make his bed.
Later, Scrooge and his staff discuss the trouble they’ve had with the ghost, trying to pass it off as an illusion or something of that nature. Shortly after that, Scrooge finds the Duchess snooping around a hallway. Unbeknownst to Scrooge, she’s apparently looking for something. The ghost then continues to scare Duckworth and Scrooge, and tries to help Mrs. Beakley and Webby as they make dinner. While snooping around some more, the Duchess runs into Benzino, telling him she’s just looking for the ice machine. Meanwhile, the nephews are scared by the ghost again, while another ghost (a visible one), spies on them.
At dinner, everyone talks about the ghost being a nuisance. The Duchess suggests that they simply ask the ghost what he wants. But by doing so, the ghost takes an axe and cuts the dinner table in half with it. The nephews want to leave, but Scrooge insists on dealing with the ghost somehow. The nephews, who think that the ghost is Benzino because of his absence every time it has appeared, insist on looking for clues, and start by looking in the nearby graveyard for Dr. Strangeduck’s tombstone. They are unable to find it, and are soon scared off by a cat, who snarls at the other spying ghost.
After snooping around some more, the Duchess accidentally reveals a hidden doorway to Scrooge. The doorway leads downstairs into the basement, so Scrooge and the boys go down to check it out. At the bottom of the stairs is a tunnel, at the end of which is what they believe to be Dr. Strangeduck’s laboratory. When the door opens up on its own, which scares off the nephews, Scrooge is determined to stop the ghost himself, but ends up getting caught by what appears to be the ghost, in the form of a shadowy skeleton.
Soon the ghost ties up Scrooge to a chair, and leaves him in the lab. After finding a way through the door, the boys enter the lab and untie Scrooge, and then explore the laboratory a bit. Meanwhile, the ghost has gone upstairs, and has stolen some of the Duchess’ jewels. The Duchess tells Dr. Strangeduck to stop, and that she’s here to help him. The ghost tries to escape from the castle, but Duckworth won’t let him lower the drawbridge.
Back in the lab, Huey discovers some sort of spray paint, which can render anything it touches invisible. With the paint in hand, Scrooge and the boys head back upstairs, where the ghost and Duckworth are still fighting. With the drawbridge lowered, Huey then sprays it with the paint, making it invisible. When the invisible drawbridge is raised, the ghost tries to escape, but he crashes into it. Scrooge and the ghost start fighting with each other, and in the process, the drawbridge is lowered, and they roll onto it. Because they can’t see the edge of the drawbridge, Scrooge and the ghost fall into the moat.
The boys then tell the Duchess that Scrooge caught Benzino, but she and Benzino himself prove them wrong. When Scrooge pulls the ghost out of the water, his paint has been washed away, and the Duchess reveals that he is really Bernardo, Dr. Strangeduck’s assistant. She also reveals that Dr. Strangeduck is her brother. The spy ghost then reveals his identity, the real Dr. Ludwing von Strangeduck.
Afterwards, the Duchess talks about how she was looking for her brother all this time, and how Bernardo was snooping around as a ghost only to find Dr. Strangeduck’s book of secret formulas. Dr. Strangeduck states that Bernardo was unable to find the book, because it was covered with his invisible paint. But with the help of his invisible paint remover, he reveals the book to Scrooge. Scrooge suggests that the Strangeducks produce the formulas in the book, while Scrooge markets it for them. Scrooge and the Strangeducks then are scared off when the book starts moving on its own, but the nephews discover that it only moved because of a rat that was hiding underneath it. They nephews decide not to tell Scrooge about the moving book, and let it be their little secret.
- Benzino Gasolini would reappear in the episode "Top Duck".
- One of the tombstones reads H. G. Duck. This is a reference to science fiction writer H. G. Wells, whose works include the story The Invisible Man, which this episode is reminiscent of. However, the dates on H. G. Duck's tombstone (1894-1943) are different from the actual dates of H. G. Wells (1866-1946).
- The nephews accuse Benzino of being the ghost, because they never saw him and the ghost at the same time. But during the dinner scene, Benzino is sitting at the table with the nephews when the ghost attacks.
- This episode has a few elements in it from Carl Barks' story "The Old Castle's Secret", such as the invisible spray and also that it happens in a castle, and others from his story The Phantom of Notre-Duck, notably for Doctor Strangeduck's disguise.
- DuckTales: Volume 1