"The Golden Armory of Cornelius Coot!" is the twentieth episode of Season 2 of DuckTales.
Webby leads the kids in a race against the Beagles to find a secret treasure while hiding a secret of her own. Meanwhile, Della is forced to teach Launchpad how to actually fly a plane.
At Fort Duckburg, there is a tourist attraction dedicated to Cornelius Coot, where a historian tells Huey, Dewey, Louie, and Webby of his heroic deeds using his saber and musket fire from all directions to defeat and scare off the Beagle Militia. After founding Duckburg, Coot made a mark on history few could ever achieve. To celebrate the occasion, Della, one of Cornelius's descendants is chosen to sky write the letter C (Scrooge charges by the letter), to commemorate his actions. The tourist attraction only sells traditional eatery and drink such as saltwater jerky and buttermilk. After Dewey churns some butter, the boys want to take part in Della's skywriting ceremony, but Webby instead suggests they search for a secret room known as the Golden Armory of Cornelius Coot, a place even Della could not find.
No sooner does Webby wish to uncover the secret, but the Beagle Boys led by Ma Beagle arrive disguised as 18th century people, eager to obtain the treasure first. Meanwhile at McDuck Manor, Della makes her way to the hangar to prepare the Sunchaser for the upcoming event, only to find it in shambles as Launchpad is attempting to fix it. He tries to assure Della that the Sunchaser always manages to be repaired no matter how many crashes its experienced, but Della begins to grow weary because of the majority of the crashes being Launchpad's fault.
Back at Fort Duckburg, Huey attempts to convince Webby that the golden armory is likely just a myth, but Webby still remains confident in solving a mystery that she claims could make them legends. Meanwhile as Ma Beagle was discussing the plan with her sons, Big Time arrives with the hopes of helping them. However, Ma instead decides to kick Big Time out of the Beagle Boys due to his prior history of failures to the group. Later as Dewey accidentally knocks over Louie's cup of buttermilk onto the statue of Cornelius, they discover a secret entrance hidden behind the metal plate attached to the statue.
Unfortunately as the kids moved forward on the path, they are not aware that Big Time notices their discovery, as he decides to follow the group with the hopes of making a name for himself to the other Beagle Boys. Meanwhile as Launchpad continues to fix the plane the way he typically would, Della instead follows his routine and begins fixing it the proper way, after realizing his way was improper in cases such as using a hamster to activate the propellers. Meanwhile, Webby and the triplets continue to search the cave for the treasure, but they accidentally trip into the wall of a moss-like substance and fall further down the cave.
Dewey then believes Webby turned off her phone light and is tickling him, only to realize there are large spiders on his eyelids, and the rest of the group (aside from Webby) freak out about spiders crawling all around them. Meanwhile, Della finally finishes up the repairs to the Sunchaser, but requests Launchpad to not touch the plane with the belief that he is a bad pilot due to his history of crashing. Nevertheless, Launchpad decides to be Della's co-pilot with the hopes of learning more about flying from Della herself after the latter suggests taking a few flying lessons; much to her annoyance and reluctance.
Meanwhile after recovering from the spiders and growing further doubts about the adventure, the kids discover a mine cart to move through the cave faster, much to Dewey's excitement. Suddenly, Big Time runs by them while also freaking out about the spiders, as he then jumps onto one of the carts and moves into the mines. Realizing that the Beagle Boys are after the treasure again, the kids follow him on the other cart, only to realize the tracks are too steep for either of them to proceed further. Instead, both parties decide to push their carts to the top before the other makes it.
Back on the plane, Della starts to have trouble teaching Launchpad about the mechanics of the plane from what he's used to. She instead suggests to him to keep staring out the window to make sure they're still in the air, when in actuality she doesn't want to be bothered by him. Back in the mines, the kids are able to make it to the end of the track, but Big Time unluckily ends up in a spider-infested tunnel while following his cart's tracks. As they continue to walk on foot, they eventually make their way towards a room with a sign confirming it as the Golden Armory of Cornelius Coot.
Although the group was originally excited to have found it, they soon discover the only materials in the room are stacks and barrels of corn. Huey then confirms from old blueprints in a nearby journal that the minecarts were actually corn carts, and the marks they found along the way were really simple farming tools. This leads the triplets to believe that Cornelius really was just a boring farmer, and his secret treasure was merely a bunch of old corn in the end. But although Webby attempts to convince them there is still more to the legend, she is interrupted when Ma Beagle and the other Beagle Boys capture them.
As the boys admit to them that the treasure was simply corn, Webby still strives to stop them from entering the room with the belief that there was still another hidden truth afoot. Meanwhile after Della accidentally sets off a button that covers the plane in smoke, she is unable to see the view. This leads Launchpad to control the plane instead and crash it in a nearby cave, where they then realize the kids and the Beagle Boys are currently fighting each other in that same cave. In order to save them, Della snatches a nearby rope and ties up the entire Beagle Boys group.
After realizing Webby wasn't with them anymore, Della orders the boys to stay where they are as she follows the tunnel to the armory room to find her. But just then, Big Time shows up from above the tunnels using a string of spider webs, and now refers to himself as Bug Time after deciding to gang up with the cave spiders and attack the intruders. Meanwhile as Della finds Webby in the armory room still trying to look for a hidden treasure, she notices her old journal slipping out of her backpack and begins to question her actions. Webby confesses to Della she had grown envious of all the adventures she went on and conquered by the time she was her age, and had hoped to solve the one mystery she couldn't which was the legend of the Golden Armory of Cornelius Coot.
But after realizing it was only corn, she believed herself to be an imposter who couldn't be the next Della Duck. Della then cheers her up by stating how she made it farther than anyone else ever has, and she only needs to be herself because she has her own unique and special ways of doing things, which then leads Della to realize she should apologize to Launchpad. But during their talk, Webby's candle accidentally ignites a wire and sets off a chain of events that leads the room to start filling up with popcorn. Webby then realizes Cornelius' true strategy of fending off the Beagle Boys from the legend, which was heating up corn to make it sound like he was firing from all ends like an army, and thus outwitting the Beagle Boys.
However, the room begins to overflow with popcorn, leading Webby and Della to flee from the armory. Meanwhile as the triplets, Launchpad and the other Beagle Boys are now strung up and about to be attacked by Big Time's spiders (which he dubbed the Buggle Boys), Webby and Della arrive and use a nearby scythe to cut down the triplets and Launchpad. As the wave of popcorn begins coming their way, Della, Launchpad and the kids board the plane while Big Time strings up the other Beagle Boys into a ball to protect them from the impact. When Della can't seem to get the plane to move, she finally allows Launchpad to use it in his own way in order to escape, as Launchpad then orders his hamster (who is revealed to be super strong) to move the propellers with his hamster wheel.
They then crash their way out of the cave and fly off, letting the Beagle Boys escape as well with Ma allowing Big Time back into the gang; much to his delight despite Ma having spider eggs in her mouth. Back at Fort Duckburg, the triplets are seen attracting tourists towards the attractions, with Della and Webby happy that they solved the mystery of Cornelius Coot's golden armory together. Launchpad then arrives and hands them each a glass of buttermilk, but neither of them enjoy it, and Launchpad's glass turned out to only have spiders coming out of it.