During a lively New Year's party celebration, Ben Matthias, an astrophysicist deeply engrossed in developing a camera to detect dark matter, encounters Alyssa, the ghost tour guide. Captivated by her strong belief in the supernatural, Ben soon marries her. Tragically, after Alyssa dies in a car crash, Ben abandons his career and takes over her ghost tour, desperately searching through infamous haunted locations for signs of her presence. However, his quest leads to despair as he finds no evidence of ghosts, resulting in a heavy sense of loss weighing on his heart.
Years later, Gabbie, a widowed doctor, along with her son Travis, relocates from New York to the enigmatic Gracey Manor, intent on transforming it into a charming bed and breakfast. To their dismay, they discover that the mansion is thrumming with ghostly inhabitants. In a twist of fate, Ben is contacted by Father Kent, an exorcist priest, who recruits him for a photographic investigation of the spectral residents at Gracey Manor. Initially skeptical, Ben's own haunting experiences with a ghostly mariner compel him to revisit the mansion.
Together, Ben, Gabbie, Travis, and Kent realize they are not alone in their hauntings, prompting their return to the manor. They join forces with Harriet, a psychic, and challenge the historical knowledge of Professor Bruce Davis to uncover the mansion's secrets. They stumble upon a hidden séance room where Harriet makes contact with the spirit of William Gracey, who communicates a crucial message directing them to seek out the renowned medium Madame Leota. However, before they can proceed, they are interrupted by an ominous entity that forces Harriet out of the house, and soon after, Bruce too experiences ghostly disturbances.
Determined, the group decides to stay at the mansion and search for Leota. Ben's adventure in the attic leads him to encounter the phantom bride Constance Hatchaway and discover a locked trunk. Inside, they uncover a crystal ball housing Leota, who reveals the tragic history: William Gracey had procured the mansion and employed her in a sorrowful attempt to connect with his deceased wife, inadvertently allowing the mansion's spirits to claim residency. After a malevolent entity emerges, tricking Gracey into his own demise, Leota finds herself trapped within the crystal confines.
In a dramatic escalation, Harriet attempts astral projection to gather more insights but instead sends Ben's spirit out of his body, leading to an encounter with Gracey's ghost and the sinister spirit of Alistair Crump—a top-hatted figure with a notorious reputation. The following day, Bruce leads Ben to the local police station to sketch the ghost, unveiling Crump's dark legacy of revenge against society after a troubled upbringing.
Despite Crump's efforts to confine them within the mansion, Ben, Kent, and Travis manage to escape. They venture to what remains of Crump's estate, now relegated to a historical site, where they learn from the Mariner that Crump has ensnared nine hundred ninety-nine souls and seeks to claim one final victim to escape and fulfill his vengeful desires.
As Ben’s group races against time, they return to Gracey Manor where they rescue Gabbie, Harriet, and Bruce from Crump's sinister grip, only to find Crump incinerating a crucial element for their banishment ritual. Disguised as Travis's late father, Crump tries to manipulate the boy's grief to claim him as his last soul. In a heart-wrenching moment, Ben persuades Travis to sever ties with his father's memory as the group confronts Crump in the graveyard, joined by Kent who rallies the spirits to their side.
Meanwhile, Bruce delivers the remnants of the banishment hat to Harriet, who courageously initiates the ritual. When Crump proposes that Ben must willingly perish to reunite with Alyssa, Ben reveals his acceptance of her death as the ritual culminates in Crump's return to the netherworld.
In a heartwarming conclusion, many of the ghosts decide to stay harmoniously with Gabbie and Travis at the manor. Ben resumes teaching, and during his departure from his apartment, he pets an amiable cat named Tater Tot, a charming reminder of Alyssa's fondness for the snack. The group warmly reunites on Halloween, celebrating their bond and newfound paths: Harriet gains confidence in her psychic abilities, Kent transforms into a genuine priest, Travis earns a spot on his school's student council, Gabbie becomes a dedicated member of New Orleans General, and Bruce cherishes his revitalized friendships.
