Thomas, Twilight, CatDog and the Great Parent Mystery is an upcoming Thomas/MLP:FIM/Nickelodeon crossover to be created by Stuingtion.
It's the big Parents' Day celebration in Nearburg, but Cat and Dog are left out because they don't have any parents, so they watch from a fence. Dog remembers the wonderful times they had with their Mom and Dad but Cat tells him that the only thing he can remember is that they've been on their own and it's been that way since then. Dog tells Cat that the day that they will reunite with their parents has finally come, but Cat tells him that they will never find them. After they got home, Dog told Cat to not be upset because Parents' Day happens every year. Then Dog plays some keys on a piano which leads to his Parents' Theory Room where he stored some balloons that he took from the Parents' Day celebrations. Dog told Cat that their parents have tried looking for them but they encountered many perils which stopped them from doing so, however, Cat says that they don't care about them anymore, but Dog was still adamant that they do care about them and that they will go looking for them. Later that night, Cat secretly enters a secret room under their bed containing his blankie and pacifier, the only mementos he has of their parents. Cat says to himself that they might be looking for them, but then he says he's kidding himself and goes back to sleep.
The next morning, there is a news bulletin saying that Old Uppenchuck is due to erupt in the next 24 hours. Winslow remarks that the last time Old Uppenchuck erupted was when CatDog came to Nearburg, and then there is a flashback of the day when Winslow first met CatDog, and then Winslow said that it was the best day of his life. Cat asked Winslow why he never told them in the first place, then Winslow said that they never asked him to begin with. Cat ponders this and deduces that he and Dog were from Yonderland, a vast wasteland where Old Uppenchuck is. The minute he mentions that, Dog asks if they're going there to look for their parents and then Cat agrees to go looking for their parents for his sake. As they are driving, Cat explains to Dog that they aren't stopping for anything until they get there. Meanwhile, Winslow has the house all to himself and when the Greaser Dogs show up, Winslow told them that CatDog wasn't home and that they went to Yonderland to look for their parents.
Back with CatDog, Dog annoys Cat with sock puppet versions of their parents. Afterward, Dog tells Cat he needs to use the bathroom and then they stop at a rest area and then encounter aliens returning everyone they ever abducted, but then get abducted themselves and get locked in the ship Cat tries desperately to open it but it was no use suddenly Aliens come to the hallway and CatDog were forced to hide in the hole in the hallway CatDog sees a light of their parents' shadows and races down the hallway they saw some people who got abducted unfortunately the get caught by the aliens. CatDog run to the control room and blasted off into space, and because of Dog's stupidity he fools around with the controls witch causes the ship to go out of control the aliens got scared and head to the escape pod, Winslow shows up but are dragged by the aliens thinking it's a "Earth Creature Trick" CatDog got shot back to Earth and The Greaser Dogs See CatDog upside down and Crashes. However, they narrowly escape into a frozen lake, where they encounter Bessie the Lake Monster is about to gobble up CatDog and eats them
and as they're being digested. Cat comes up with a plan to escape by making a hairball and succeeded. As they escaped, the Greaser Dogs attempt to pursue them but are gobbled up by the monster. As CatDog lands, their fur returns and they landed on a very sharp rock. Dog sniffs around looking for signs that their mother and father may have been there and then Cat, having been getting tired of Dog's theories and detours, tells him that they are going straight to Yonderland, until they come upon a sign for the town of Yokelburg, home of two happy hillbilly clans, the McDogs (a clan of dogs) and the Catfields (a clan of cats), who are celebrating 50 years of peace between each other.
CatDog then thinks that Hound Dog McDog and Pussycat Catfield are their parents, but it turns out that they are actually the parents of Lube. However, during the celebrations Cat drinks some moonshine and burps in front of Pussycat Catfield's daughter. Unfortunately for Cat, the custom in Yokelburg is when you burp in a lady's face, that means you want to marry her. Cat refuses to marry her, and that instigates a feud between the Catfields and McDogs. the Catfields and McDogs were about to begin their feud and the Greaser Dogs watch from the McDogs' side, and CatDog blame each other for the feud. The fighting continues until both the Catfields and McDogs run out of ammo to use against each other, forcing both sides to engage in close combat until the entire town ends up a wreck because of the excess weight on CatDog, who escape on a log. Meanwhile, the Catfields and McDogs continue to blame each other for the destruction their town, but then Cliff tells them that CatDog is to blame for their feuding. Meanwhile, back with CatDog, they are on a raging river and then they get down a waterfall
CatDog finally arrived at Yonderland, where Dog goes running looking for their parents, but was sad and disappointed in not finding them. After a well deserved lecture from Cat, Dog finally agrees that looking for their parents was stupid,dumb and silly, and that he wishes he was more like Cat, but then Cat said that he wishes he was more like Dog because Dog was determined to find their parents and never gave up hope looking for them, but then they were taken by a big, blue four-eyed monster into a cave and then the monster says Dog's catchphrase, and Dog immediately recognizes her as their long-lost mother, but Cat says it has four eyes and then the second pair of eyes reveals itself to be a frog with a big nose and CatDog's father. Dog hugs the frog, but Cat said that they weren't a CatDog, but this was because they were their adopted parents. Then they flash back to the wonderful times they had together, culminating in that horrific day when they got separated from each other. Then CatDog's mother explains that they were looking for them ever since then, and then CatDog's father proves that most of the theories that Dog had mentioned in the previous episodes were true. Cat and their father then recognized each other and hugged, and then Cat showed the other half of their pacifier and Cat started to cry, and then they embrace with their mother when Winslow shows up and gets startled by CatDog's parents and CatDog's mother gets startled by Winslow and accidentally squishes their father and then Dog introduces Winslow to their mother and father, and then when Winslow was sad because CatDog wouldn't be around anymore, Dog offered to let him stay with them, but he refused and told him that the Greaser Dogs, the McDogs and the Catfields came to pound them and managed to back them into a corner when a tornado showed up and then Cat had no choice but to overcome his fears and then tie himself and Dog to their parents and then they escaped back to Nearburg. The Greaser Dogs, Catfields, and McDogs afterwards were eaten by Bessie. The Movie ends with CatDog and their parents celebrating their own Parents' Day and their new lives in Nearburg.
- Zecora, Stephen, Sharky, Mako, Willy, T.C., Hugs, Sherin, Buzz, J.J., Socky, Skunky, 1206, The Miner Trains, Evan, Steamy, Puffy, Shai-Shay, Rattlesnake Jake, The Planet Trains, Dusty Crophopper, Ishani, Bill and Ben, Duke, Mrs. Calloway, Maggie, Grace, Norbert and Dagget, Timber, Splinter, and Chomper, Falcon, Vanellope von Schweetz, Spencer, Duck, Oliver, Toad, Stepney, Donald and Douglas, Stanley, Arthur, and Murdoch guest star in this film.