Donald is happy at a letter he has just got. When Huey, Dewey, and Louie arrive trying to tell something, they find their uncle in a good mood and decide to listen his news first: the letter says he has won the Villa Rosa mansion in the lottery, and it included the property key. When the nephews point he hasn’t bought any lottery ticket, Donald is about to tell them his name was on the envelope until he notices it actually was meant for Gladstone Gander, though he hides this fact from them.
When Donald prepares to leave to Villa Rosa, he finds what the nephews wanted to tell him; they found a lost puppy. Donald angrily throws the puppy away before driving the 313 towards Villa Rosa, and Gladstone finds the puppy. While Donald drives, he reads to his nephews that the owner of Villa Rosa died without leaving heirs and one hundred years later, the mansion became government property, and then was donated to the city hall which donated it as the big prize of the lottery. When the four ducks arrive to Villa Rosa, it’s full of dust and webs, which scares Huey, Dewey, and Louie away, and only Donald remains there. He finds under the couch a book labeled “Secret diary of Fantomius”. He hides the book and tells the nephews they should rather return home.
In the way back home, Donald asks his nephews what they know about somebody named Fantomius. Dewey tells him he was a gentleman thief who stole from the rich. The 313 suddenly stops before crashing with a limo driven by Gladstone, who got it for returning the puppy to his tycoon owner and wins a huge pay for puppy-sitting.Back at home, Uncle Scrooge is awaiting Donald to offer him a job: giving him wind with a fan while he reads. Donald refuses and drives Scrooge away. When the nephews criticize him for first driving away a tycoon’s dog and now refusing a job, Donald drives them away and, finally alone, begins reading the diary of Fantomius. It mentions, among other things, a car full of special devices. Donald takes that specific page and goes to see Gyro Gearloose.
At Gyro’s house, Donald is surprised to find Uncle Scrooge, but it’s actually Gyro who was testing some carnival masks with perfect likeness. Donald asks him to add the page devices to the 313, and in the meantime, he uses Gyro’s bi-reactor scooter, taking the masks with him.
Donald goes to Villa Rosa, where he finds a hidden disguise and a hypnotic lamp. He goes to Scrooge’s house and gives him the lamp as a gift. Back at home, after dinner, Donald dons the disguise for the first time and becomes the Duck Avenger! What is his plan?
Scrooge goes to read at bed, but doesn't know the lamp Donald gave to him emits hypnotic fumes that make him sleep.
The Duck Avenger arrives at the McDuck estate, uses a spring seat to pass the fence (tough he's still hurt by a fox trap) and uses a special key that melts the locks. Once inside, the Avenger cuts te alarm. The money is easy to take, so it doesn't present enough challenge and the Avenger decides to take Scrooge's mattress from under him. then he uses a corrosive spray to melt the front door and leave.
On his way to Villa Rosa, the Duck Avenger is seen by two cops who follow him, but he uses the new tricks of his 313-X to lose them. However, they notice he goes to Villa Rosa. There, the Avenger is found by the cops, but he knows every hidden trick of the place, and uses these tricks to escape and go to a hidden hideout. When the cops leave, the Avenger notices they lighted a candle that doubles as dynamite, so he turns it out and leaves to his house.
The next morning, Scrooge awakens Donald for the mattress robbery, as he and Gladstone are suspects. As the cops know the mattress has to be somewhere in Villa Rosa, everybody goes there. Donald creates a distraction and puts the mask of his own face to the puppy Gladstone is sitting, so Scrooge deduces the thief was wearing that mask to frame Donald, who acusses Gladstone as his puppy was carrying it. Scrooge stops their fight to seek the thief. Donald wears the Scrooge mask to misguide Gladstone while he goes ahead to Villa Rosa. However, Gladstone arrives first and Donald decides to return home to await his sad fate.
At Villa Rosa, Gladstone lights a candle and finds the mattress. However, it was the dynamite candle, which explodes destroying Villa Rosa and throwing Gladstone to the air alongside the $1,000,000 in small bills the mattress had.
Later, at Donald's home, Scrooge and Dewey tell him they found Gladstone was the thief and Scrooge won't place charges if Gladstone collects the bills. Donald just tells "Call me vengeance" which nobody understands.