The Secret(s) of Castle McDuck! is the twenty-first episode of the Disney XD animated series, DuckTales.
On a family trip to Scrooge's ancestral castle, Dewey tries to hide the truth about his mother as his brothers search for family secrets.
Launchpad drives Scrooge, Webby, Huey, Dewey, and Louie through a mist-shrouded area of Scotland. Using the Sphere of Selene, Dewey views images of his mother Della Duck in the past. Webby then confronts him about the fact that he hasn't yet told his brothers about what he's learned about their mom. He claims that it's so they won't get hurt, but ignores Webby's attempts to convince him that it makes more sense to tell them the truth.
Launchpad crashes the jeep outside a large, ominous looking castle; Scrooge reveals that they are in Dismal Downs, and that the castle is the home of the Treasure of the Knights Templar. Suddenly, the front door opens to reveal Fergus and Downy McDuck, Scrooge's parents. Downy is happy to see her son and great-grandsons, but Fergus and Scrooge's reunion is cold. Scrooge then reveals that his parents are still alive due to him using druid stones to rebuild Castle McDuck.
Fergus and Downy take the boys on a tour of the mansion, while Webby is overcome by the sheer amount of Clan McDuck history in the place. The boys then discover a note hidden on the back of a portrait of Donald, seemingly giving clues to a treasure hidden within the castle. Upon realizing that the note is in Della's handwriting, Dewey tries to dissuade Huey and Louie from getting involved, but is unsuccessful. The trio end up in the catacombs beneath the castle, which are guarded by a monstrous Demon Dog.
Scrooge sits down to dinner with his parents as he continues his efforts to convince Fergus that he is worthy to know the location of the Templar treasure. However, his claims to being a family man are doubted due to the absence of both the boys and Webby, who is unable to cope with being in Scrooge's ancestral home. Hoping to win Fergus over, Scrooge runs outside and gets Launchpad, who he ordered to park the jeep. He dresses Launchpad up as Donald, but Launchpad's inept performance only succeeds in pushing Scrooge and Fergus to blow up at each other.
The boys continue to make their way through the catacombs, with Huey and Louie using their talents to advance the quest while ignoring Dewey's attempts to get them to turn back. They eventually discover the family crypt and a chamber reserved for Donald, which the note has been leading them to. Inside, they find a duffel bag that Dewey realizes belongs to Della and which contains various personal effects of hers. However, they are then attacked by the Demon Dog, and to prevent his brothers from sacrificing the items for a distraction Dewey tells them the truth about his quest.
Huey and Louie are understandably upset by Dewey's revelation, as Dewey kept it for himself to feel special. Dewey realize he’s been selfish for doing this. the Demon Dog breaks trough, and end up running in separate directions through the catacombs. As the Demon Dog pursues Dewey, Scrooge sets out to find the treasure himself, only to run into Fergus. The two eventually come across a large vault, and Scrooge asserts that he has never needed Fergus, citing his Number One Dime as an example. He is stunned when Fergus reveals that he was the one who caked Burt the Ditchdigger's boots with mud and sent him to young Scrooge with the dime as payment for his services.
Fergus finally admits that his ill temper around Scrooge is due to missing his son, as he inadvertently set Scrooge on a path that led to the two being separated. Reconciling, the two then enter the vault, only to find that the treasure has been moved by Fergus' father Dingus McDuck, who left a note expressing the same negative opinions of Fergus that Fergus has been voicing about Scrooge. Meanwhile, Dewey is cornered by the Demon Dog, but Huey and Louie come to his defense, citing their bond as being more important than any disagreement. They then join together to scare off the Demon Dog, unaware that they are joined in their efforts by several members of Clan McDuck.
The boys then discover that the treasure Della's note hinted at was a hat she took from Donald, and that the note was planted as a prank on him. They then resolve to keep looking for answers together, with Dewey further admitting that Webby has been in on the quest as well. When they look on the backside of the note from Della's prank, Huey shades the backside of the paper which reveals a faded picture of the Spear of Selene and dates on the right side. Scrooge and Fergus similarly resolve to search for the Templar treasure together when Scrooge is able to return in five years' time. Fergus and Downy bid their goodbyes to Scrooge and his party, with Webby regaining her ability to speak too late to ask Scrooge's parents any questions.
Notably, Toks Olagundoye (Beakley) and Tony Anselmo (Donald) are mistakenly credited instead of Graham McTavish (Fergus) and Ashley Jensen (Downy).
Absent: Donald and Mrs. Beakley
Scrooge mistakenly refers to Donald as Fergus and Downy's great-nephew, when he is actually their grandson.
The story of Fergus making sure the Number One Dime got to Scrooge comes from the comic story Of Ducks, Dimes, and Destinies.
Huey, Dewey, and Louie's birthday is revealed to be during the week of April 15th, which was the release date of their animated debut in the 1938 short Donald's Nephews. This, however, contradicts the comic story "Donald Duck in Panama" (originally printed in Walt Disney's Vacation Parade #4), in which they mentioned that they are Leos, which would put their birthday between July 22nd and August 23rd.
Alan Young, the voice actor for Scrooge McDuck in the 1987 Ducktales series was originally planned to be asked to voice Fergus McDuck for this episode, but he passed away before recording preparations came to be.