In the quaint town of Friendship, Maine, the eerie Whipstaff Manor becomes the latest possession of a neurotic and self-centered woman named Carrigan Crittenden. This inheritance comes with a twist; as explained by her father's lawyer Rugg, an invisible ink inscription in the will hints at a hidden treasure within the manor’s haunted walls. This revelation piques the interest of Carrigan and her attorney, Paul "Dibs" Plutzker, igniting a series of comical and spooky events.
Upon their arrival, they encounter Casper, the friendly yet lonely ghost. However, their shock quickly escalates into chaos as Casper's three mischievous uncles—Stretch, Stinkie, and Fatso—make their presence known. In desperation to rid themselves of these spectral nuisances, they summon numerous professionals, including the flamboyant Father Guido Sarducci and the ever-reliable Ghostbuster, Raymond Stantz, but none succeed in vanquishing the ghosts.
Finally, they engage James Harvey, a therapist adept at communicating with the deceased. Harvey is accompanied by his teenage daughter, Kat, as they seek solace in their new home while still haunted by the memory of his late wife, Amelia. A heartwarming yet tumultuous series of events ensues as Casper, enamored with Kat, strives to win her friendship. Unfortunately, his uncles’ rambunctious antics present constant challenges, leading to a hilarious vacuum cleaner showdown.
The next morning finds Casper making breakfast for Kat and Harvey, laying the groundwork for camaraderie. At school, Kat's reputation is sealed as her classmates decide to host the upcoming Halloween dance at the infamous Whipstaff Manor, an idea that sparks jealousy in Casper when he learns of a schoolmate, Vic, asking her out. Meanwhile, Harvey uncovers the secrets of Casper's uncles and learns their antics are fueled by glee rather than malice.
Through thoughtful discovery, Kat finds Casper’s cherished childhood toys in the attic, an endeavor that helps rekindle his fragmented memories. This newfound recollection culminates in a touching act where Casper gifts Kat his mother's dress for the dance. As Casper digs deeper into his past, he recalls the tragic circumstances surrounding his own death, involving a sled and a fatal illness due to winter revelry. Alongside Kat, he decides to search for a long-lost machine created by his father, which supposedly resurrects the dead, embarking on an adventure that unravels deep emotional layers.
Meanwhile, Carrigan and Dibs continue their fruitless quest for the treasure, ultimately coming across a vault within the very lab Casper and Kat explore. Rather than working together, their selfish ambitions lead Carrigan to an unexpected fall, returning in ghostly form to seize the treasure herself, a baseball signed by Duke Snider, which holds sentimental value for Casper.
In a twist of fate, after Harvey accidentally meets his demise amidst the chaos, Casper is at a crossroads between his own desires and the needs of others. With an act of selflessness, he uses precious resources meant to restore himself to bring Harvey back to life. In a parallel celebration, the arrival of guests at the mansion ignites a party filled with ghosts and laughter, showcasing how unconventional friendships can flourish in extraordinary circumstances.
Amidst the merriment, Casper gets a brief reprieve from his ghostly existence, transforming into human form to dance with Kat, symbolizing hope and companionship. However, as the clock strikes ten, Casper must return to being a ghost, but not before leaving a lasting impression during the festivities. Ultimately, Kat, Casper, and Harvey solidify their bond, choosing to continue the party with joy and laughter, while the echoes of Casper’s uncles singing add to the enchanting atmosphere.
