On a family trip to Scrooge's ancestral castle, Dewey tries to hide the truth about his mom as his brothers search for family secrets.
Scrooge brings the kids on a sojourn to Dismal Downs, Scotland for cryptic reasons (and without telling Donald). Knowing the area holds secrets of their family, Dewey confides in Webby about keeping his brothers from uncovering more on Della Duck, fearing a revelation could upset them. When the group comes to Castle McDuck, home of the Clan McDuck, the family is greeted by the lairds of the estate — Scrooge's parents, Fergus McDuck and Downy McDuck.
At the castle, Downy greets Scrooge and the kids with open arms. Scrooge explains that years ago, Scrooge helped buy back and rebuild Castle McDuck with mystical Druid stones. This deal backfired for his parents, for they were cursed with immortality, only reappearing along with the castle once every five years when the mists of Dismal Downs part enough to reveal it. They guard the Lost Treasure of the Knights of Templar hidden by Simon McDuck, therefore Scrooge travels there whenever the castle returns to claim it, but Fergus will not relinquish it unless Scrooge proves he is worthy of it.
While exploring the family portraits, Dewey keeps vigilant of any clues on Della before coming across a riddle written by her. Huey and Louie theorize it will lead them to the treasure and decide to embark on it. Using the first clue, they find the entrance to one of Sir Swamphole McDuck's passageways, though Dewey attempts to solve the riddle before his brothers do.
Scrooge and Fergus remain tense towards each other during the visit as the latter suspects Scrooge's irresponsibility towards family. After Scrooge and Fergus get into an argument during dinner, Scrooge sets off to explore the castle on his own to claim the treasure, having had enough of trying to prove himself to his father, although Fergus tags along, too.
In the catacombs, the triplets unearth the crypt of deceased McDucks where they find one tomb designated to Donald. Inside, they find only a bag of Della's belongings right before they encounter Swamphole's demonic Hound. While fleeing from the Hound, Dewey is forced to admit that the bag belongs to Della. His brothers react severely to Dewey withholding the truth, Huey reprimanding him for keeping it for himself and Louie becoming devastated. Dewey realizes he has been selfish for doing this.
During Scrooge and Fergus' search, Fergus reveals that the Number One Dime came from him to instill Scrooge his value of hard work; had he not, Scrooge wouldn't be the successful duck he is today. Scrooge sees Fergus is right thus he apologizes to his father at last. What they expect is the treasure, at last, turns out to be a letter from "Dirty Dingus" McDuck, having found the treasure first and hid it somewhere else. Although they didn't find the treasure, Scrooge and Fergus, having mended their grudge, vow to find the real treasure the next time Scrooge returns.
Somewhere else, Dewey is on the run from the Hound. His brothers hear Dewey's call for help and apology and return to save him. The spirits of their ancestors come to their aid, and together they scare the Hound away. The riddle then leads them to the final mark: Donald's hat, as part of an elaborate prank left by Della. The triplets then decide to continue the search for Della together, starting with another clue Huey finds on the back of the riddle: a sketch of what appears to be some manner of aircraft with a date that coincides with the week the boys hatched.
The family then warmly parts ways with the McDuck couple. But, to Webby's horror, she was so overwhelmed by the McDuck history that she forgot to ask the couple about Scrooge himself after they leave.
- David Tennant as Scrooge McDuck
- Danny Pudi as Huey Duck
- Ben Schwartz as Dewey Duck
- Bobby Moynihan as Louie Duck
- Kate Micucci as Webby Vanderquack
- Beck Bennett as Launchpad McQuack
- Graham McTavish as Fergus McDuck
- Ashley Jensen as Downy McDuck
- Scrooge introduces Launchpad as Donald to his parents as their grandnephew. However, Donald is actually their grandson. The mistake could be attributed to nervousness on Scrooge's part.
- This episode is loosely based on the Carl Barks story "The Hound of the Whiskervilles" and its 1987 DuckTales adaptation, "The Curse of Castle McDuck". Like its 1987 counterpart, the Hound is an actual creature, though it is not the Whiskervilles'.
- Additionally, its plot takes inspiration from stories of the Uncle Scrooge comics, specifically "A Letter from Home" (Fergus hiding the Knight Templar treasure), "Of Ducks, Dimes and Destinies" (Fergus giving Scrooge the Dime), and "The New Laird of Castle McDuck" (Scrooge buying back his family's land and the ghosts of Clan McDuck). Ironically, in the latter, it was Fergus himself who couldn't pay the taxes.
- According to their animated debut in the 1987 series episode "Once Upon a Dime", Scrooge's parents are deceased; near the end of the episode, when Huey asks if Scrooge considered himself rich when he built his Money Bin, Scrooge replies sadly: "Oh, just the opposite, lad. Without my parents, I felt poorer than ever".
- Unlike Once Upon a Dime, Fergus and Downy are alive in the present day here.
- Dewey finally tells Huey and Louie about their mother in this episode.
- The triplets going down the staircase is a nod to a similar shot from the original series episode "Hotel Strangeduck".
- The ghosts of the McDucks consist of Sir Eider McDuck, Sir Swamphole McDuck, Seafoam McDuck, Molly Mallard, Sir Roast McDuck, Quagmire McDuck, and Sir Simon McDuck.
- Although Donald and Bentina Beakley aren't in this episode, their voice actors are still credited.
- Fergus's first line in the episode was the same line Scrooge said to Donald in "Woo-oo!".
- Huey, Dewey, and Louie's birthday is revealed to be within a week of April 15, the same date as their animated debut in Donald's Nephews.
- This, however, contradicts the comic story "Donald Duck in Panama" (published 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.
- Launchpad disguising himself as Donald is a reference to the fact he was originally created to replace Donald in the original DuckTales.