The Ghosts are the supporting antagonists of the horror movie, Poltergeist - angry and confused, these souls latch onto the life force of a young girl known as Carol Anne after having their original gravesite violated by the careless actions of Lewis Teague and are further controlled by "the Beast" (an alias of Reverend Henry Kane) into causing havoc on the mortal plane.
The ghosts never show themselves in person but manifest in many frightening ways, most notably with poltergeist-activity (hence the name of the film). They also begin to employ increasingly dangerous and diversionary tactic including actual kidnapping while isolating other members of Freeling family. What began as a seemingly innocuous paranormal activity from ghosts soon turns into outright hostility when ghosts abduct Caroline Ann Freeling into their dimension, inflicting lots of griefs to Freeling family in the process. The ghosts also create several "minions" to do their bidding - such as the Clown Doll and the Gnarled Tree, though Reverend Henry Kane likely had some input in these events due to his exceptional power as a demon.
Desperate to reclaim their youngest daughter, Freeling family calls parapsychologist trio named Lesh, Ryan and Marty from University of California, Irvine for help. When the three parapsychologists arrive to Freelings residence, the poltergeists have taken a great toll on the family. After series of paranormal episodes and more hostile acts from ghosts, remaining Freeling children are sent away for their safety. Lesh informs Freeling family, that she is leaving with Ryan to bring a reinforcement but Marty, who are petrified after being the most victimized of the trio by ghosts' abuses, would not be returning. Later that day, Mr. Freeling is approached by his boss, Lewis Teague, about a promotion. The new project will involve selling lots on a newly acquired hilltop parcel of land that currently houses a cemetery. When Mr. Freeling balks at the idea of relocating the graveyard, his boss simply shrugs it off, explaining that the company had done it before, in the very neighborhood where Freeling family now resides.
Lesh returns with a renowned spiritual medium named Tangina Barrons, a diminutive person who immediacy Steve is doubtful about her ability, and is rather quickly proven that he's wrong. Tangina informs Freelings, Lesh and Ryan that Carol Anne is alive and in the house, much to everyone's relief. And Caroline's life force is very powerful, but that life-force of hers is keeping the ghosts from passing on. The souls for whatever reason are not at rest. They look to Caroline's living presence as a way of remembrance of their lives, and the joy it gave them.
Tangina then explains about the entity behind the darker deeds taken place in the Freeling house. While ghosts have been mostly not bad, the sinister deeds (the skeletal hand, the growling wind, the possession of the tree, and Marty's attacks etc.) have been orchestrated by what Tangina calls 'The Beast': a powerful spiritual entity which has been corrupting other ghosts to use their spiritual skills for power, enticing them with the promise of the presence of Carol Anne.
The reassembled group discovers that while the entrance to the other dimension is through the children's bedroom closet, the exit is through the living room ceiling. After hatching a childish but very feasible plan the group sends Mrs. Freeling to bring back Carol Anne. It was an incredibly risky procedure that almost killed Carol and her mother, but while Carol is being taken away from the clutches of ghosts Tangina coaxes ghosts away from Carol Anne and convinces them to pass on to the afterlife. Carol Anne is once again at the hands of Freelings. Tangina announces that the spirits are gone.
However, although most ghosts moved on to the afterlife, the Beast did not. Wanting to exact vengeance after losing the life force of both the ghosts and Carol Anne, on the family's final night (when children returned) in the house the Beast ambushes Freeling family once more. Locking the children in their room, attacking Robbie with the thing he fears most, The Beast then isolates Mrs. Freeling. After breaking free she makes her way to the children's room. But the Beast blocks the door to the room. In a panic, Mrs. Freeling tumbles down the stairs while The Beast manages to electrify the railings to prevent her from coming back up. Mrs. Freeling runs to her neighbors screaming for help, only to slip and fall into the unfinished swimming pool, from which coffins and rotting corpses erupt: these corpses are the original bodies of ghosts in the movie. The neighbors, terrified by the unnatural and ghostly energy emitting from Freeling house, refuse to help. Meanwhile in the children's room, the closet has imploded and turned into a monstrous structure that threatens to suck in Carol Anne, attempting a second shot of kidnapping at Carol. Mrs. Freeling nonetheless manages to pull her children including Carol Anne out from the imploding house. Freeling family in haste fled from Cuesta Verde without looking back.
In the end of the movie, the ghosts take it upon themselves to finally put an end to the torment by literally spewing forth the contents of the gravesite while also causing the Freeling's home to implode and thus purify their sacred site again: destroying a significant portion of the nearby community in the process. These spirits achieved their own retribution in the end.
But lately, another group of spirits among the ghosts who are completely and they simply followed the will of the Beast. In fact these spirits are the losts of a Satanic, 19th century utopian cult who will be still after Carol Anne.