The Great Celestrian Observatory is a movie spoof fanfic and the first movie spoof fanfic feautring most of the entire cast being video game characters in history, written by sasagirl. It also features two characters from her file from the Nintendo DS game Dragon Quest IX: Senitels of the Starry Skies, Lavigne, her main protagonist, and Donna, one of the three premade party members in the game, as Basil of Baker Street and Major Dr. David Q. Dawson, respectivly. It also spoofs the Disney movie The Great Mouse Detective. The story started on May 22nd, 2012, and finished on June 1st, 2012.
- Lavigne (sasagirl's version of Dragon Quest IX: Sentinels of the Starry Skies) - Basil of Baker Street
- Donna (sasagirl's version of Dragon Quest IX: Sentinels of the Starry Skies) - Major Dr. David Q. Dawson
- Gunge (Fraggle Rock) - Olivia Flaversham
- Philo (Fraggle Rock) - Hiram Flaversham
- Marjoly (Rhapsody: A Musical Adventure) - Professor Ratigan
- Nyanko, Nyanchi, and Nyanki the Nyankos (Rhapsody: A Musical Adventure) - Fidget
- The King of Cornelia (Final Fantasy I & II: Dawn of Souls) - Queen Mouestoria
- Angel (sasagirl's version of the Nintendo DS version of Dogz) - Toby
- Klesta (Final Fantasy Tactics A2: Grimoire of the Rift) - Felicia
- Mollie (Dragon Warrior VII) - Mrs. Judson
- Other Nyankos of the Marjoly Family (Rhapsody: A Musical Adventure), Maya (We Cheer 2), Ai (We Cheer and We Cheer 2), Kaitlin (One of the fanmade characters on the cheerleading squad that Maya leads made by sasagirl on We Cheer 2), Lumpy (Happy Tree Friends), Cuddles (Happy Tree Friends), Toothy (Happy Tree Friends) Mittens Fluff 'n Stuff (Lalaloopsy), Bundles Snuggle Stuff (Lalaloopsy), The Old Man Raposa character (Drawn to Life), Coach Summers (Imagine: Figure Skater), Ildon (SaGa Frontier), and Reiha (Summon Night: Twin Age) - Various Extras, including Bartholomew
Following a similar plot to The Great Mouse Detective, some things are different in the story.
In Stornway circa a current year, Philo and Gunge are spending time with each other in a cottage. Suddenly, Nyanko, Nyanchi, and Nyanki, three humanoid cats known as Nyankos, burst into the cottage, kidnapping Philo. It's later revealed that Marjoly kidnapped Philo to create a clockwork robot which mimics the King of Cornelia so Marjoly can rule Stornway. Philo refuses to take part in her scheme, whereupon she orders the three Nyankos to capture Gunge and make him a prisoner.
Then Gunge searches to find Lavigne of Celstrian Observatory Street, a world-famous detective and Marjoly's archnemesis. Donna stumbles upon Gunge, and helps her find Lavigne's residence. At first, Lavigne is reluctant, but when Gunge mentions the three Nyankos kidnapped Philo, she then realizes that this is her chance to capture Marjoly. Lavigne and Donna then use a Pomnerian puppy dog named Angel to track the three Nyankos' scent to a nearby toy store. The three Nyankos are surprised by Lavigne, Donna, and Gunge in the toyshop where they are stealing clockwork mechanisms and toy soldiers' uniforms for Marjoly's plan. They then hide and later trap Gunge by ambushing him from inside a toy cradle. Lavigne and Donna pursue the three Nyankos, but become entangled in some toys and fall behind, giving the three Nyankos enough time to escape with all the materials they need, along with Gunge.
While searching the shop, Donna discovers the three Nyankos' forgotten checklist, which details everything they have taken with them. Lavigne and Donna return to Celestrian Observatory Street, where Lavigne discovers by means of close examination and some chemical tests that the list came from the riverront, and they look for a small tavern near somewhere where the sewer would connect to the river. Lavigne and Donna then disguise themselves as sailors and go into the tavern, inquiring for Marjoly. As they wait, Nyanko, Nyanchi, and Nyanki stumble through the pub. The two follow them through some pipes to Marjoly's headquarters, only to discover that Marjoly and all of the other Marjoly Family Nyankos had prepared for their arrival. Triumphant, Marjoly then ties them to a spring-loaded mousetrap, connected with a Rube Goldeberg machine made of a gun, crossbow, axe, and anvil. Marjoly then sets out for Stornway Palace, as the Nyankos kidnap the King. Lavigne briefly despairs at being outwitted, but snapos out of it just in time to deduce the trap's weakness and escape.
Back at Stornway Palace, Marjoly forces Philo to operate the toy King, while the real King is being taken by the three Nyankos to be fed to Klesta, Marjoly's Crushatrice friend. The fake King then declares Marjoly ruler of all Stornway, and she announces her tyrannical plans for her new "subjects". Just then, Lavigne, Donna, and Gunge save Philo and the real King, and apprehend all of the Nyankos, there. Lavigne then seizes control of the mechanical King, forcing it to denounce Marjoly as an impostor and tyrant, all the while breaking into pieces. The crowd, enraged by Marjoly's treason, start climbing onto him and defeating his shanty guards. Marjoly then frees herself and escapes on a dirigible with the three Nyankos, and holding Gunge hostage.
Lavigne, Donna, and Philo create their own craft with a matchbox and some small hellium-filled balloons, held under the Union Jack. A high-speed chase above the city ensues. Marjoly throws the three Nyankos into a river to "lighten the load" while on the way to their deaths, and then attempts to drive the dirigible herself. Lavigne then jmps on to the dirigible to confront her; however, with no helmsman, Marjoly is unable to steer her craft, and it ends up crashing straight into a clock tower. Inside the clock, Lavigne and Marjoly then face off in a final battle. Lavigne rescues Gunge and safely delivers him to Philo, who is still in the baloon with Donna. Marjoly plunges into Lavigne, and they both fall onto the clock's hour hand. The fight ensues, and Marjoly begins to vicously thrash her. It seems as if Lavigne's luck is about to run out until the clock bell tolls and Marjoly plunges off the hand taking her with her, however, Lavigne saves herself just in time.
Back at Celestrian Observatory Street, Lavigne and Donna recount their adventures as well as the King's gratitude to their saving his life, and afterwards, Philo and Gunge leave to go back home to help out Marjory the Trash Heap give advice to various Fraggles that need help with problems that they can't solve. Donna figures it's time for her to leave as well, but the scene is interrupted by a distraught new client. Lavigne then persuades Donna to remain as "my trusty associate, Donna, with whom I do all my cases".
- This is the first movie spoof fanfic story having most of the cast to be video game characters in history, and sasagirl's first movie spoof fanfic to write.
- Unlike The Great Mouse Detective, which takes place in London, England, this story takes place in Stornway from Dragon Quest IX: Senitels of the Starry Skies.
- Marjory the Trash Heap herself is not seen in the story, but is mentioned a few times.
- Unlike one scene of The Great Mouse Detective, which features Fidget sleeping before Ratigan announces at him, this scene of the story features the three Nyankos not sleeping, but instead features Nyanko and Nyanchi playing a board game, and Nyanki reading a The Five Little Fraggles book before Marjoly announces at them. The Five Little Fraggles is also a story that Marjory tells or reads to Philo and Gunge every and some other night before bedtime in Fraggle Rock, as seen in the episode Home is Where the Trash Is. This is also the first sasagirl movie spoof fanfic that has an appearence in the same story.
- Unlike a birthday party scene in the beginning of The Great Mouse Detective, there is no birthday party scene in the beginning of the story, and instead, it features Philo and Gunge spending time with each other in a cottage. Also, a wind-up ballerina toy was for Olivia in the same scene of The Great Mouse Detective, and even a locket for Gunge in the same scene of the story, as well.
- This is the first movie spoof fanfic that features Lalaloopsy characters as extras.
- Unlike the scene where Basil feels despress while playing his violin while laying back in his chair in The Great Mouse Detective, this scene of the story features Lavinge feeling despressed while relaxing in a hammock in despair, instead of playing a violin, guitar, or piccolo.
- This is the first sasagirl movie spoof fanfic to have various Marjoly Family Nyankos being incorporated into the story.