What Ever Happened to Donald Duck?! is the seventeenth episode of Season 2 of DuckTales.
Donald and Penumbra must break out of a Moon Prison and send a warning to Earth about a looming invasion. Meanwhile, Dewey and Webby uncover a sinister conspiracy targeting their family. 
On the Moon, Lunaris continues to fill his fellow Moonlanders with hatred for Earth. He then claims to have captured an earth spy which confirms the Earth's prior plan to invade them first, as Penumbra is then seen dragging Donald, the supposed Earth spy, towards Lunaris and the rest of the audience. Lunaris then clips Donald's beak shut with a golden strap, as he goes on to explain that Penumbra is the one to thank, having been the first to find out about Della's plan to invade them and thus promotes her to captain. Penumbra wasn't comfortable with the idea of Lunaris lying to their people to proceed with the invasion, but Lunaris states that now she is considered a hero because of it. When he asks if that's what she's always wanted, Penumbra agrees even with her conflicted feelings.
Meanwhile on earth, Scrooge is in his office assembling a miniature money bin inside of a bottle, when Dewey suddenly bursts into the room claiming he uncovered a conspiracy. Dewey originally believes there to be a secret fourth child among him and his brothers after noticing a strange spot on a photo of them as eggs, but Scrooge points out to him that it looked more like a mustard stain. Webby also comes barging into the room after her own discovery of a family tree painting that turned out to show geographical coordinates to a treasure. However, Scrooge assures her of having already found the treasure as he points to a nearby knight armor.
Scrooge then tells them that an adventure must call to them, rather than going around making your own mystery. Webby then states that having spent most of their time on the mystery of the Spear of Selene, they were eager to move on to the next big mystery they could find. Webby and Dewey then overhear Huey speaking to himself in the hall, due to his postcard to Donald having been oddly returned to him. This encourages Dewey and Webby to start their next mystery, but as they leave the room, Dewey accidentally slams the door so hard it knocks over Scrooge's miniature money bin.
Back on the moon, Donald witnesses all of the rockets being built in preparations for the invasion, as he is transported to the Tranquility prison. Donald attempts to escape from his cell by using his head to break a small hole into the wall, but he is interrupted when Lunaris and Penumbra are seen walking towards his cell, and attempts to cover up the cracks with a nearby poster. Lunaris states his confidence in the invasion preparations to Penumbra, but Penumbra herself begins to question if their people are truly prepared to go to war, having almost no combat experience and information about Earth. Nevertheless, Lunaris shrugged it off by recalling their motivation chant, "we are the mighty, we are the moon."
As Lunaris leaves, he then orders Penumbra to move Donald to work below the gold mines, in the hopes that he loses his fighting spirit through hard labor. Once Lunaris is gone, Penumbra opens up the cell as Donald proceeds to make numerous attempts to flee from her, but fails in doing so. Penumbra then admits her own rejecting thoughts to Donald about the invasion plan, and considering it was Donald's family who created the rockets, she hoped Donald would know a weakness to them to stop the invasion. Even though Donald didn't know, Penumbra still considered his shaking head as an Earth yes.
Meanwhile, Dewey and Webby begin to question Huey about the unreceived postcard to Donald, as they eventually lead to the conclusion that he's gone missing. Huey, however, believes it is simply hard to receive mail from a boat and he was spending most of his time relaxing. Even so, Dewey and Webby proceed to search for further clues about Donald's whereabouts. Back on the moon, Penumbra reveals to Donald that Lunaris keeps the rocket's blueprints under guard in his private hangar. But as they're making their way there, they are spotted by fellow Moonlanders Zenith and Gibbous. When asked why she let Donald out of his cell, Penumbra lies by claiming she was using Donald as practice for fighting the other Earth inhabitants, encouraging them to hit Donald for practice as well.
Once Zenith and Gibbous leave, Penumbra orders Donald to use a nearby vent and make his way over to the hangar, while also avoiding the soldier's barracks. Back on Earth, Dewey informs Webby he plans to send both of them to the address marked on the postcard while inside a large package. But as the mail lady comes, Dewey realizes he accidentally forgot to tape the bottom shut, and thus reveals themselves the moment the mail lady lifts the package. Dewey then begins to question her about Donald's whereabouts, but after the mail lady realizes the mansion was where Donald now lived, she hands them numerous letters originally addressed to him at the pier, unaware that he switched addresses.
After noticing the bills are marked with the same address, they decide to investigate the address, believing Donald was on the run from a debt collector. Back on the Moon, Donald continues to make his way through the vents while also beginning to crawl away from a large Moon bug chasing him. During which time, he accidentally falls through another vent and lands on a bed within the soldier's barracks, as he tries to stay silent as possible without waking anyone up. Meanwhile Dewey and Webby approach the building of the address marked on the bills, in the hopes of finding the supposed bill collector Jones trying to find him.
Dewey then suggests a distraction for Jones by having Webby dress up in a Junior Woodchuck uniform (which he stole from Huey) pretending to sell cookies, while he sneaks into Jones's office to find any information about Donald. Dewey then sneaks in through the window, but just as Jones was about to open the door, Webby knocks on the front door to lead him away, which then allows Dewey to discover a large file dedicated just to Donald. Meanwhile Donald attempts to make his way out of the room while trying to keep all the soldiers asleep, but his plan abruptly fails after the bug chasing him jumps into his shirt and causes him to scream.
Although all the Moonlanders in the room are alerted, Donald is luckily able to fend them off due to the bug crawling inside his shirt, leading him to have numerous uncontrollable spasms and attacks on the soliders. Penumbra eventually enters the room to find out what the commotion was about, only to realize Donald had already taken care of all the soliders, impressing her. Donald and Penumbra then make their way over to Lunaris' hangar to find the blueprints, but Penumbra warns Donald not to touch anything as Lunaris would indefinitely find out. This brings Donald to lean on one of the walls out of frustration, only for him to discover a secret war room hidden by the Moon's holographic technology.
As Penumbra activates the controls, they notice numerous screens appearing of locations and citizens among Earth, including Donald's family, leading Penumbra to believe he's been planning the invasion since before Della even arrived on the Moon. Donald then realizes Lunaris plans to target all the inhabitants of Earth he deems a threat to the mission first, while Penumbra realizes that Lunaris had secretly been creating test rockets prior to meeting Della, which is why he truly wanted Della's rocket manual to begin with. Meanwhile as Dewey is looking through Donald's file, Webby regroups with him after having sent Jones to search through his drawers over a phony locket quest.
Dewey and Webby then realize the file shows every date and record of all the bad things that ever happened to Donald, only to be approached by Jones himself and thrown into the closet while preparing to make a call. Back on the Moon, Penumbra orders Donald to send a message to his family to warn them about the attack using Lunaris' own transmission invention. Suddenly, Penumbra is electrocuted and knocked unconscious by the captain badge Lunaris placed on her earlier. Lunaris then shows himself and reveals he chose to come prepared, in the event Penumbra decided to rebel against him.
Even so, Donald flees from Lunaris while striving to get high enough so he can get a signal to send a transmission. However, Lunaris continues to block his path, and although Donald tries to fight back, the results are futile. Meanwhile, it is revealed that Jones phoned Scrooge to come and get Dewey and Webby. Scrooge also reveals to them that Jones is actually Donald's anger management counselor, who's sent Donald bills to pay for his time with him. When Dewey then questions Jones on why Donald is still the angriest person they know, Jones explains that Donald's anger comes from a fear that the world is out to get him, and that no one can understand him figuratively and literally
Overtime his tendency to lash out became uncontrollable, until his nephews came along and encouraged him to go to Jones in order to be a good parent figure. Jones taught Donald to channel that anger into a protective instinct for his family. He then states that Donald loves his family so much, the thought of anything bad happening to them would infuriate him beyond control. Meanwhile, Lunaris then gloats about his plans to target Donald's nephews and thus striking the spirit of one of his greatest threats, Scrooge McDuck. This leads Donald to become filled with rage, as he is then able to fight Lunaris well enough to allow him to send a warning to Scrooge about the coming invasion in one month.
But just as the message is sent, Lunaris snatches the transmitter from him and destroys it. In order to escape, Donald makes his way into Lunaris' testing rocket, as it then shoots him away from the Moon. Lunaris decided not to follow him, believing there was no way anyone could survive the test rocket's trip, with Penumbra remarking the bravery of Donald once she regained consciousness. Meanwhile, Scrooge, Dewey and Webby leave the office with Scrooge shouting at Jones about his refusal to pay for Donald's bills. Although Webby admits it wasn't a good idea to go looking for a mystery when there wasn't one, Dewey reminds her that they did find out the mystery of why Donald was mad at the world, because he loves them.
Just then Scrooge receives a voicemail from Donald about his warning, but Scrooge is unable to fully understand what Donald is saying due to the poor reception, while only understanding his claim to be back home in a month. The scene ends as Donald is seen still inside the rocket and flying within the atmosphere towards an unknown location.
Dewey theorizes that a fourth sibling named Phooey might exist based on what looks like a fourth egg (which is really only a mustard stain) in childhood photos of them as eggs. Phooey would later appear as a dream construct in A Nightmare on Killmotor Hill!
Donald trying to escape via a hole in the cell wall and then hiding his work behind a poster is a reference to The Shawshank Redemption.