Della struggles for survival against dangerous elements of the Moon, as she looks for a way to get back to her family on Earth.
10-11 years prior to the Shadow War, Della is flying into the cosmic storm with the Spear of Selene, overconfident about her chances. Scrooge contacts his niece via transmission, trying to turn her back before the storm kicks up when suddenly, a bolt of energy hits the Spear of Selene, sending the rocket and Della spiraling down to the moon. Della is flung from the cockpit by the aftershock and knocked unconscious. A day after the crash, she wakes up with her helmet now cracked and on the verge of suffocation, when suddenly she spots a package of Oxy-Chew invented by Gyro. She takes a piece and chews it, giving her oxygen, but is disgusted by the black licorice flavor. While struggling to get her leg free, she notices her copy of the photo of herself, Scrooge and Donald and three eggs- which would soon be her sons flying down from the wreckage. With the broken rocket booster about to crash down on her, Della says one of her family's catchphrases- "Aw phooey". Days later, Della has no other option but to amputate her left leg to free herself, replacing it with a metal one from the parts of the Spear of Selene.
Two months after the crash, Della attempts to use the ship's transmitter to communicate with Earth and let her family know that the storm caused most of the ship's systems to fail but she was alright. She is able to activate a distress beacon in order to hopefully point anyone who could be looking for her in the right direction. She also explains her current living style, and attempts to draw out what she thinks her newborn sons would look like as she was unable to see them hatch. She leaves a message to them saying it is only a temporary situation, and she will be back before they even know it. Her recording is then interrupted when a giant bug monster known as a Moon Mite starts attacking her ship, but she luckily survives.
Nine months after the crash, Della begins to grow weary that no one is coming to save her after she receives no response to the transmissions she sent out over the months. She reveals to the camera that she brought her Junior Woodchuck Guidebook with her, and plans to create ideas from the book for communicating back to Earth. She then attempts to create messages from the metal scraps of the Spear Of Selene for someone to see it on Earth, but this ends in failure after the Moon Mite returns and destroys her messages with its corrosive saliva.
One year after the crash, Della makes an attempt to wish her sons a happy first-year birthday with gooey rubble on the moon shaped like a cake, and a flare as a candle. Soon after, Della spots one of Scrooge's ships looking on the moon for any traces of Della. Della attempts to get close to the rocket, but is blocked once again by the Moon Mite. Unfortunately her attempts to have the ship notice her end in failure as her brawl with the Mite creates a large dust cloud. Because of this, the pilot of the rocket ship naïvely mistakes the cloud formation for a meteor strike on the surface of the Moon and departs, leaving Della in tears. She then decides to create a new plan - to fix the Spear of Selene itself. Although her first few attempts fail due to ignoring the rocket manual's instructions, she eventually gives in and decides to do everything by the book.
Six years after the crash, Della is successfully able to repair the Spear of Selene and attempts to finally launch it, but the launching sequence fails. Della soon realizes that it's because Gyro invented the ship's engine to be powered by gold itself, but unfortunately there is no gold available to use. This leads Della to embark on a journey to find gold on the moon, believing that if there was any, she will find it.
Ten years after the crash, Della believes she has searched every spot on the Moon for gold, only to turn up with nothing, angering her and making her blame Scrooge for not leaving any spare gold to use in case the engine ran out. Della begins to get frustrated after having been away from her family for so long, as well as chewing the same disgusting gum the whole time, and eventually her anger leads her to break a tooth. However, she soon realizes that the tooth itself is made out of pure gold, leading her to believe the times Uncle Scrooge took her to the dentist was to make sure she always had a little bit of gold on her at all times. This brings hope to Della once more as she believes she can use the tooth as fuel for her rocket ship's engine.
Soon after, however, the Moon Mite returns again and begins taking apart Della's ship once more. Della attempts to flee from the Moon Mite after realizing it was after her gold tooth, but is knocked down and about to be killed until bolts of electricity are shot at the Moon Mite, causing it to flee. Della then meets the two aliens who saved her that went by the names of Penumbra and Lunaris, who are among a race known as Moonlanders that live on the Moon. They then go into the underground caves of the moon where the Moon Mite has also taken Della's ship with it.
They eventually run into the bug monster and try to fight it off, only for the Mite to spit acid onto Della's ship, burning away most of the metal, seemingly to the point where it can no longer function. This infuriates Della, which leads her to run at and fight the Moon Mite hands-on, only to be interrupted by the sounds of baby-like squealing coming from inside the ship. Della then comes to understand that the squealing sounds were coming from the Moon Mite’s baby, and it was only attacking them in order to find proper metal to feed her and her baby. This leads Della to willingly give her golden tooth to the Moon Mite, knowing that a mother would do anything for the sake of her kid.
Even so, the baby refuses to eat the golden tooth their mother was trying to offer. Della then tries to cheer the baby up by singing a lullaby she used to sing to her boys before they hatched.
Look to the stars, my darling baby boys, Life is strange and vast, filled with wonders and joys. Face each new sun with eyes clear and true, Unafraid of the unknown, because I face it all with you.
The baby then cheers up and is able to eat the golden tooth, with Della also giving the mother and her baby the metal weapons Penumbra and Lunaris used to attack them, for extra nourishment. The mother and her baby then leave them peacefully, as they and Della wave goodbye to one another before they burrow deeper underground. Lunaris praises Della for staving off the Moon Mites with words instead of violence. However, Della seems to resign herself to her fate, fearing she will never make it home with her ship in its current ‘unrepairable’ state. However, Lunaris offers her a home within the Moon, saying that even though she can't get back to her own home, she can create a new one where they stand.
This brings Della to the discovery of an entire civilization of the Moonlander race known as Tranquility, as they had kept the city hidden on the moon using a hologram in order to stay hidden from other aliens and Planet Earth. Della becomes infuriated after realizing she had been "slumming" it in the moon desert for 10 years, yet also overcome with joy after realizing their main resource in the city is gold, giving her further hope that she can still have a chance fix her ship again and return back to Earth and her family. Behind Della's back, Penumbra shares her frustration with Lunaris after seeing him allow her to have access to their civilization and resources. But Lunaris reminds her that Della was able to defeat one of their greatest enemies as if it was nothing, which leads him to believe she could be useful in the upcoming future. Nevertheless, Penumbra still disagrees, believing Della to be a danger to them, and silently mumbles to herself while referring to Della to watch her back.
This is the first episode to not feature much of the main cast, save Scrooge for a few moments.
This episode's plot and video-oriented framing device bear many similarities to the 2015 film The Martian. Like this episode, that film is about a person from Earth who becomes stranded alone on an otherwise uninhabitable other world and must survive by their own wits and expertise, continually recording their own video journals and hoping someone on Earth will notice they're still alive.
Penumbra saying "Halt, in the name of the Moon!" is a reference to Sailor Moon's "in the name of the Moon" speech.
This episode reveals the existence of a civilization of aliens called 'Moonlanders' who live on the Moon in a hidden city.
Although actress Paget Brewster provided the voice for Della in prior episodes such as "The Shadow War!" and "Last Christmas!", this was the first episode she began recording for to get a better understanding of the character and her voice, rather than starting off with a younger version of her character. 
Entertainment Weekly placed "What Ever Happened to Della Duck?!" among the 30 best episodes of 2019
The episode was originally named "They Put A Duck On The Moon".