Duckburg, beginning of the XX century. Fantomius and Dolly Paprika are escaping the police after robbing from a rich lady. Fantomius activates a smokescreen to lose the police and returns to his secret hideout in Villa Rosa. When they are already home, Dolly notices she lost her pendant with the key of their stolen treasure. Since then, Fantomius' treasure remains there, awaiting to be recovered.
Present day, Scrooge McDuck is reading about that same legend when he's interrupted by Brigitta MacBridge, who wants to invite him to a costume party with a scavenger hunt. Scrooge is uninterested until he hears the prize for the winner is Dolly paprika's pendant.
The day of the party, Daisy tells Donald she will go disguised as Dolly Paprika and he should dress as Fantomius, but Donald would rather use another disguise as Fantomius' suit is too similar to the Duck Avenger's one, and it may raise suspicions on his identity. Scrooge goes to Donald's House and tells him to guard the Money Bin while he's at the costume party, and that he has already told to Daisy, who then decided to go with Gladstone Gander instead.
At night, while Donald guards the Money Bin, he finds Scroge's book about Fantomius' treasure, and goes to his secret hideout to confirm that info with Fantomius' diary (from the first Duck Avenger story) and even finds some info not known by Scrooge.
The Duck Avenger goes to Gladstone's house and puts a "car-stopper" on his car, which will make it stop each five minutes. After this, the Duck Avenger dons a Fantomius mask to go to the party pretending to be Gladstone.
At the party, Scrooge forges an alliance of wits and luck with "Gladstone" to win the scavenger hunt. The first hint is "it has long age and its lights are out, you will find him over the sea". The Avenger deduces it's the old lighthouse. There, they find the note "it sings the hours punctually with its old voice, if you believe in ghosts it's their home". Daisy deduces church bells "sing" each hour and Brigitta tells a ghost is rumored to live in the Notre Duck church.
Scrooge must pay to enter the church, and only when he has already paid the guard tells he had the hin the whole time. It says "where firewood doesn't burn anymore, you will find the treasure, if you know how to find the gentleman thief's lair". That means it's Villa Rosa, Fantomius' old home.
They go to the ruins of Villa Rosa and the Duck Avenger deduces "where firewood doesn't burn anymore" is the chimney, and Scrooge finds Dolly Paprika's pendant there. Daisy tells they should return to the starting point to tell they won, but Scrooge finds a map carved in the pendant, telling about a trapdoor. Daisy is scared to go down, so the Duck Avenger tells she's a coward unworthy of Dolly Paprika's costume, so he manages Daisy to be angry at Gladstone.
Scrooge and the Duck Avenger go down while Brigitta and Daisy remain outside. They find a painting of Dolly and the Avenger deduces the key is to put the pendant right on the painting's pendant. However, there's no money or anything, and while Scrooge cries, the Duck Avenger leaves through another secret trapdoor. Right then, Gladstone arrives, but Daisy is angry for "his" earlier comment, and Scrooge comes out asking him the golden pendant, so he can get something from all this. When Gladstone tells he doesn't know about any pendant, Scrooge chases him believing he's trying to steal it.
Back at his secret hideout, the Duck Avenger re-reads in the diary the part Scrooge didn't know: Dolly returned after the pendant, but the place was full with policemen and she returned sad to Villa Rosa, but then Fantomius told her that he had another emergency key and after her loss of the pendant, he moved the money to other vault. Fantomius wants Dolly to keep the key of the new vault: a compromise ring.