While the triplets are watching an Ottoman Empire marathon, Mrs. Beakley enters the room to clean up the plates they left lying around. When she moves into the kitchen to wash the dishes, she is attacked by a hooded assailant whom she eventually recognizes as Black Heron. Beakley puts up a fight within the kitchen to defend herself, but is nevertheless knocked unconscious with tear gas by her attacker.
The next morning Scrooge comes downstairs and notices the boys still watching the Ottoman Empire marathon, as he joins Webby in the dining room for breakfast. Scrooge then questions Webby about Beakley's whereabouts due to her absence in giving him his daily nutmeg tea. This gives Webby the opportunity to make the nutmeg for him herself, but although Scrooge is satisfied with his tea, he starts to feel on edge when Webby begins mimicking his every move due to her years of study on his life. Webby and Scrooge later enter the kitchen to grab Scrooge's breakfast, only to find it in disarray.
Scrooge dismisses Webby's suspicion that Donald was attempting to cook again, as he is heard shouting while locked in the cupboard following his last effort. Scrooge then comes across a black feather, which he alarmingly recognizes as same feather belonging to an old foe. A flashback shows a younger Beakley in S.H.U.S.H. headquarters, where she was to receive a debriefing from Director Ludwig Von Drake, but had already familiarized herself with the mission details.
She explained that Black Heron was plotting to steal the last page of the "Great Book", it which it was recently seen at Castle Dunwin. Not only is the page said to contain dangerous information, but both Beakley and Ludwig believed Heron to also be working for F.O.W.L. Beakley also learned that the page was planned to be used at an auction later that night, and considering it was a high-class event, they managed to get a VIP invitation from a wealthy freelance operative. However, to her dismay, Ludwig informed Beakley that she would be working with the freelance operative himself Scrooge McDuck, who was fresh off his defeat of Jack the Tripper.
Ludwig then directed their attention to the equipment they needed for the mission, in which he opens a briefcase containing what appeared to be a normal cane similar to Scrooge's. Ludwig explained how it was no ordinary cane, in which it was his own invention he dubbed the "Von Drake Action Cane PPK", equipped with a number of unique gadgets for the mission. During the demonstration, Scrooge however ended up being careless with the cane and accidentally activated one of the gadgets. Back in the present, Scrooge bids his farewells to the boys as he is leaving the mansion to find Beakley, only for Webby to show up with her own supplies for the mission.
Webby demands that Scrooge takes her with on the rescue mission to save her grandmother, but Scrooge orders her to remain in the mansion, believing Beakley would want her safe and away from harm. Later on a submarine in the ocean, Scrooge recovers Von Drake's modified cane while the driver of the submarine Launchpad notifies him that they will arrive in a couple hours. During the ride, however, Scrooge realizes Webby had snuck onboard after discovering her in the live vest bin. Webby then admits her prior awareness of him and Beakley's past adventures, along with Beakley's previous status as an agent known as "Agent 22."
Although Scrooge wants Webby to stay away from danger, he still prefers to take Webby with him over leaving her in a submarine controlled by Launchpad. Back in the flashback, Scrooge and Beakley were present at the auction as Beakley began to question what was on the page from the Great Book. Scrooge stated that the page contained forms of alchemy on it, with formulas used for potions and elixirs. He also stated that legend has it, the high priests of Castle Dunwin were mentored by creatures with never before seen powers. However, their conversation was put to a halt when they spotted Black Heron within the crowd.
The bidding for the page then began, but once Black Heron placed her bid up to seventy-thousand dollars, Scrooge became stingy on bidding any higher due to S.H.U.S.H. having only given him sixty-thousand dollars for the mission. Due to Scrooge's unwillingness to offer any cash from his own pocket in time, Black Heron won the page with her bid. Nevertheless, it didn't stop her from stealing the page anyways, as she then fled the auction by jumping out the window. Although she managed to escape, Beakley was able to throw a tracking device on the back of her dress so they'd know where she was taking the page to next.
Back in the present, the submarine arrives on a remote island in the middle of the ocean, where Scrooge and Webby begin making their way towards the laboratory positioned on the island. During the walk, they are approached by a mysterious orb, and although Webby is intrigued by it, Scrooge warns her to stay away from it. Back in the flashback, Scrooge and Beakley arrived on the same island and were approached by the same orb, to which Beakley points out that despite its appearance, it's actually an elite killing machine. Back in the present, Webby recognizes it as a creature who has the ability to ignite anything it lands on.
Just as the orb is about to land on Scrooge and Webby, Webby manages to poke a hole through the inside of it with Scrooges cane, and unleashes helium within it, causing the orb to float away elsewhere. This surprises Scrooge, as he recognizes Webby's technique as the same one Beakley used when they fought against the same orb years ago. He also recalls Beakley's annoyance for Scrooge's ruthlessness at his inability to follow the safety protocol. Scrooge and Webby then enter a mysterious cave, where Webby shouts out a battle cry. Back in the flashback, Scrooge had also shouted a battle cry which then caused them to get ambushed by a crew of fighting droids.
Beakley managed to twist one of the droid's head off, while Scrooge managed to throw one of the droids into a piranha pool, and causing another to lose an arm while being disabled with a hit to the head with rocks. However, despite its injuries, the last droid managed to escape. Beakley believed they should go after it, but Scrooge shrugged it off, believing they were likely to never see it again. However, back in the present Scrooge is surprised to see the surviving droid still functioning after so long during the newly started battle. But after the brief scuffle, Webby manages to kick the droid into the same pool and disables it once and for all.
Scrooge begins to be impressed with Webby's skills to the point where he questions why they hadn't done anything like this after all the years she's spent at the Mansion. Webby admits that Beakley told her to never bother him, because at the time Scrooge was always so focused on his business that he no longer had time for adventures. Scrooge is also surprised at how much Webby knows about him, while also wondering as to why he knows so little about her after so long. As the two of them continue to bond about their interests, they arrive at the entrance to the laboratory and make their way to the top using the elevator.
Back in the flashback, Beakley made her way to the top floor of Black Heron's lab, but unfortunately mistaked Heron for Scrooge in the darkness, and is then knocked unconscious. In the present, Scrooge and Webby arrive in Heron's lab and notice a vintage movie projector on the main table. The film shows Black Heron addressing herself to world leaders while Beakley is seen strapped to a table in the background. Beakley warned Heron that her back molar is rigged with a distress signal, annoying Heron due to her interrupting her speech of harmful intent.
While Scrooge sneaked his way towards Beakley from above, Heron proceeded to explain that she had obtained history's greatest biological weapon, in which she planned to create an army of super soldiers that would bounce to the highest peaks of corruption. However, her speech was once again interrupted when Scrooge laughed beside her about how she's actually making a bouncing serum, in which Heron still believed it to be a useful tactic. This eventually lead to a fight between Scrooge and Heron, but the film ends before the fight is finished. When Webby asks as to how he and Beakley managed to defeat Black Heron, Heron shows up behind them, now with a robotic right arm and points out that they didn't.
Scrooge warns Webby to not get too overconfident in an attack, but Webby nevertheless leaps towards Heron for a hit, but is instead shoved away to the ground. Scrooge attempts to warn Webby to get back up, but this results in both of them getting knocked unconscious by Heron's tear gas. When they regain conscious, they are revealed to be tied up on the floor, as they spot Beakley within a cage hanging from the ceiling. Beakley becomes outraged over Scrooge's decision to allow Webby to come along on a past S.H.U.S.H. case, as Heron then approaches them and demands silence.
She reveals that she had spent decades in hiding tracking Beakley down, rebuilding her laboratory and herself a new arm. She then claims that before she moves on to her intents of world domination, their current predicament she's responsible for is merely about revenge. Heron then demands the formula for the bouncing serum, but Beakley claims she doesn't remember. However, Heron recalls to her a certain protocol stating that all mission data must be memorized by the agent, while also holding Webby hostage to force her to comply. Unfortunately it works, as Beakley offers her the formula in exchange for Webby's safety:
Six handfuls of red berries, four orange berries, three purple berries four blue berries, three green berries, and one yellow berry... in that order.
Heron then mixes the formula together and successfully manages to create the serum, while also adding that she plans for every eventuality. She then admits that although Webby is more talented than she imagined, she intends her to be the peak of her revenge. Although Scrooge attempts to convince Heron to leave Webby out of it by belittling her, he is secretly implying to Webby to drink the serum herself. Webby then manages to kick the serum out of Heron's hands and into the air, as she lands herself at the exact spot of the potion's landing to allow her to drink the serum.
Suddenly, Webby begins to feel an uncontrollable sensation in her body while managing to untie herself, as she then begins to bounce all throughout the lab. Once Webby stabilizes herself, she unties Scrooge while Heron herself manages to drink the serum from another glass. A brief scuffle then begins between Webby and Heron with their new bouncing abilities, but Heron is thrown off guard for a moment as Webby then manages to steal back the serum. Heron then uses her robotic arm to shoot the serum out of Webby's hand, where Heron manages to recapture it.
While Webby and Heron bounce their way to the rooftop of the laboratory, Scrooge uses his cane's laser function to free Beakley from her cage. On the rooftop, Webby continues to chase Heron for the serum. Their bouncing then leads to both of them bouncing super high in the air, but once their high enough, the bouncing serum begins to wear off as they both start plummeting to the ground. During the fall, Webby manages to steal the serum back while kicking Heron away, as Beakley then throws Scrooge towards Webby to catch her. Once he safely catches Webby, Scrooge activates his cane's umbrella function as they slowly descend back onto the roof.
Meanwhile just as Heron is about to reach the ground, she catches her robotic arm onto the edge of the rooftop. When Scrooge, Webby and Beakley make their way towards the arm, they realize the robotic arm had been torn off of Heron right above the ocean water. Later, the three are now back onto the submarine making their way home, with Scrooge bringing the arm with them. Although Webby is still energetic from the aftermath of the battle, she soon asks Beakley and Scrooge about what actually happened during their first fight between Black Heron.
Back in the flashback, Beakley managed to break out of her chains and drink the entire bouncing serum, where she then managed to retrieve the last page of the Great Book and tore it up for good measure. Infuriated, Heron dashed towards her, but missed and instead caused an explosive liquid to spill across the table, and be ignited due to its mixture with the spilled bouncing serums. Beakley managed to retrieve Scrooge as he was surprised she was willing to break protocol to save his life. They then fled the burning laboratory while Heron attempted to reach with her arm for a surviving serum, only for the lab to explode just before she could get it.
Back on the outskirts of the island, Beakley believed she was likely to be cleaning up many more of Scrooge's messes in the future, as the two then greeted one another properly for the first time. Back in the present, Webby thanks Scrooge for allowing her to be a part of an "Agent McDuck 22" adventure, in which he then asks to be called uncle Scrooge as the two embrace. The scene ends as the camera zooms in on the surviving bouncing serum Webby managed to retrieve and left in her pocket.