Afterlife in Deedstown
The Ghostly Trio had been spending most of their afterlives in the world of the living, avoiding ghost training from the Ghost Central in the land of the dead, run by the monstrous ghoul: Kibosh. They've been haunting in a town called Deedstown where they inhabited an old mansion called Applegate where a group of protesters, were trying to save the place from destruction by a group of construction workers, hired by Tim Carson who wants to build a mini-mall in its place, but the Trio scared both groups away.
As they were in the mansion celebrating, the Trio were interrupted by Tim's 10-year-old son: Chris Carson who was trying to take their picture. The Trio was obviously annoyed by this, considering that it wasn't the first time this happened, and Chris was not afraid of them, being that he is a kid who is into the supernatural. Chris wanted to join them, but humans being friends with ghosts isn't in their nature, so they rejected him.
Meeting and Training Casper
When Chris re-enter the mansion this time with a mysterious someone covered in sheets, the Trio assumed at first was one of Chris's friends, but once Chris removed the sheet, it was revealed to be a small white ghost name, Casper, much to the Trio's surprised to encounter someone like themselves. They also found out that Casper hasn't gone to ghost training, which gives them the idea to train Casper in an attempt to prove themselves to Kibosh, so he would stop chasing them.
The first lesson they thought Casper was going into the stealth mode (going invisible), in which Casper manage to succeed in his first try; however, things were going south for them when Casper can't seem to pass all the other lessons they try to teach him. This led them to the conclusion that Casper doesn't have the audacity to scare others, and wants to be friends with them, which also led them the decision to kick him out stating, that there is no place in this world for a friendly ghost.
Saving a Reputation
When the Trio returned to the mansion at one night, they discovered in the newspaper that Casper is doing good deeds for people around town, so they plan on abducting him to save their reputation, but Casper distracted them to getaway.
Captured by Kibosh
Upon a night of searching for Casper, the next morning, the Trio gathered together as they were beginning to lose hope. Things manage to get a turn for the worse when they got an unexpected surprise from Kibosh, who had entrapped them in a cage and set off to find Casper.
Once Kibosh arrived at the Applegate Mansion to capture Casper, Casper's body began to grow ginormous, after eating a bomb to save the mansion that was rigged to blow, by bomber Bill Case. This managed to free the Trio from the cage.
Kibosh being impressed by Casper's technique, Casper informed him that the Trio are the ones who thought him, which confused the Trio at first, but immediately take it in just to show Kibosh that they have great techniques, so Kibosh decides to let the Trio stay, but still wanted to take Casper, so the Trio decide to return Casper the favor by saying that they are his uncles, so he can't take him after Kibosh mentions the importance of family, so Kibosh along with his assistant Snivel decided to let the four of them go in peace.
A New Home
With Casper now being the Trio's adopted nephew, they realized that they couldn't stay once learning of Tim's plan to tear down the mansion, but Tim changed his mind and decided for them to stay. Seeing they have a truce with Tim, Stretch stated that Chris can ask anything for a favor, so Chris pointed to the school bullies, who lock them in the mansion with the bomb, in which the Trio immediately accept the offer, and so they scared the bullies and have them hang in a nearby tree with there underwear in a mega wedge.
A Night at the Baseball Stadium
After a baseball team won a match against another, a mysterious spaceship arrived, and was split in three as one of them was in the form of an alien, The alien was revealed to be Fatso, where he along with Stretch and Stinkie scared the players and the citizens who were watching the game. Casper tried to calm the crowd, but they equally scared of him.
As a reward for there victory, the Trio decide to take a vacation with Casper having to carry their bags.
Vacation at Sunnybrite Resorts
Casper and the Trio arrived at a place called Sunnybrite Resorts to spend their vacation, where they scared a married couple and inhabited the cabin in which the couple was going to spend their Honeymoon. Casper isn't too pleased with this, and think they should try to be more friendly, but Fatso stated that 'a good fleshie is a scared fleshie because they can't scare back'; however, Stinkie stated that witches are an exception, due to there powers.
When the four decided to have a meal, the Trio gobble it up without offering Casper any, leaving Casper depressed much to the Trio's delight.
A Night at the Dance
After scaring away a housekeeper, Casper appeared being impress and ask if they can teach him some of these tricks, which surprise the Trio to see their kind-hearted nephew wanted to scare 'fleshies'. Casper thought about them teaching him possession, in which they mention that they are the top three ghosts, who inhabit fleshies, but Casper lied in stating that someone broken, so Casper offer a brochure to the dance to protect their title.
At the dance, they inhabit three humans and attend the dance where they encounter three girls, whom they instantly caught an attraction to. Things were going well for them until they completely lost control of the humans they inhabit and revealed their true selves to them, which scare the girls at first, but immediately reveal themselves as witches: Gert, Gabby, and Fanny, which frightens the Trio. Casper and his new friend, another witch named Wendy who was the niece of the three witches, tried to explain that it was a set up to get the six of them to get along, but they wouldn't listen as they each tried to attack each other, but the witches got the upper hand when Gert attempt to turn them in bean bags, which cause the Trio to flee away.
Back at the cabin, the Trio decided to make a stand in an attempt the scared the witches away. When Casper returned, the Trio confronted and blamed him for what happened, as Casper tried to demand them to leave the witches alone; however, the Trio suspect that he is trying to protect them, which force Casper to accidentally tell them that they can't use their powers (after Wendy made him promise that he would not tell), which gives the Trio the opportunity to get their revenge. In order for him not to interfere, the Trio flushed Casper down the toilet and proceeded.
They invaded the witches' home and began to scare them, which led Wendy to use her wand against them, and turned him into plaster, as they fearfully flee away.
Saving the Witches
They confronted Casper yet again, believing that he led them to a trap. Casper explained that the witches are on the run from an evil warlock, named Desmond Spellman (who was keeping an eye on the radar, to see if any of the witches used high-level magic, track them down, as reasons for why the witches couldn't use any magic) and stated they should help them, yet the idea of helping witches sound insane; however, Casper explains that at the night of the party they showed to have an appreciation for the witches, in which they replied that they were only pretending.
Later when the witches were being confronted by Desmond, a huge three-eyed mud monster appeared before Desmond scaring him which caused him to fall inside a swirling vortex called the Mystic Abyss in which he used to cast Wendy inside. The three-eyed monster happened to be the Trio combined as one. With the warlock defeat, the Trio teamed up with witches to get Casper and Wendy out.
Once the two were out, they were unconscious, as the Trio tried to nurse their fallen nephew up, and witches trying to do the same to their niece. Once everything turned out well, the witches thanked the Trio for saving them, as the Trio show a sign of having feelings for the witches.
After Desmond's Oracle in the mirror informed that Wendy is now the greatest witch for befriending a ghost, the ghosts and witches decide to leave with the Trio decided to do Casper's choirs for a change.