Also: Lucy Cohu as Constance Strickland, Diana Kent as Harriet Cathcart, Richard Durden as Alexander Cathcart, Cal Macaninch as Freddie Strickland, Andrew Havill as George Vandermeer, Tilly Vosburgh as Vera Flood, Steven Cree as Sergeant Evans, Alfie Field as Victor Parry, Sidney Johnston as John Franklin, Daniel Pirrie as Captain Mills, Nicholas Amer as Edgar Hirsthwit
England, 1921. Florence Cathcart is a published author on supernatural hoaxes who works with the police to expose charlatans and debunk supernatural phenomena, having begun her foray into her profession upon the death of her lover in World War I.
She receives a visit from Robert Mallory, a teacher from a boys boarding school in Cumbria with the request to investigate the recent death of a student, Walter Portman. Robert explains to her that there have been sightings of the ghost of a child at the school. Robert asks Florence for help to figure out why the supernatural events occur.
Although Florence originally refuses, Robert's concern for the orphaned kids at the school causes her to change her mind since she is an orphan herself. Florence travels to the school, which was a stately home until 20 years earlier, hoping to explain the sightings and the death.
She is welcomed by Maud, the school's housekeeper and matron, who tells Florence that she has read her books and feels like she knows her. After investigation, the ghostly sightings are at first thought to be a prank led by two boys at the school who have been bullying a lonely, redheaded boy into doing what they want for their approval.
With regards to the recent death, Florence deduces that one of the teachers was the last person to see the boy alive and questions him. He admits to making the young boy stand outside to "man up", after he claimed to have seen the ghost, thus scaring the young boy and causing him to have a fatal asthma attack. The school then closes for half-term with the only occupants being Florence, Robert, Maud the housekeeper, and Tom, a lonely boy who tells Florence his parents live in India which takes too long for him to travel to.
Her job seemingly done, Florence prepares to leave but an unexplained hand reaches for her from a pond. She faints and falls into the pond. Florence is rescued by Robert but he and Maud are concerned about her mental stability. Afterwards, the characters treat the incident as a suicide attempt. Florence recovers but starts seeing and hearing more unexplained things, which prompts her to put to rest supernatural apprehensions and stay at the school. Florence and Robert start developing a mutual attraction.
More unexplainable supernatural events start to manifest and subsequently the story is unwound through revelations. After chasing what she believes is the ghost, she sees an apparition of a man with a shotgun, who appears to shoot her. She also keeps hearing a child's voice calling "Mawa Zee", which she tells Tom was the name some Africans gave to her, after she was rescued from a lion attack as a child.
Edward Judd, the grounds keeper, earlier known to have a grudge against Robert for being a war hero, is jealous and attempts to rape Florence in the woods. Florence, assisted by a supernatural apparition, accidentally kills Judd in self-defense by the butt of his own gun. She stumbles back to the school and tells Robert, who then goes out to bury Judd and cover up the incident.
Robert is revealed to be able to see and communicate with some of his own ghosts, which form an unexplored tangent to this story. Florence tells Robert not to tell Tom about what happened as it will upset him but Robert tells her that there are no children at the school at the moment.
Florence realizes that Tom is the ghost that is haunting the school and was Florence's half brother and Maud's son. With this revelation, buried memories begin to surface in Florence. The stately home that is now the boarding school was once Florence's family home where she lived as a child. While Florence and Tom were young, it was the place where she watched her deranged father murder her mother with a shotgun and then tried to kill her and too.
Florence ran into the hidden passages in the walls of the house where she used to play but her father pursued her calling out for his "little mousie".
Aiming for her, her father then accidentally killed Tom, who was revealed to be her father's illegitimate son and Florence's half brother. Her father then killed himself while Florence watched. Maud is Tom's mother, and was Florence's childhood nanny.
Due to the intense trauma, Florence had blocked these childhood memories, including her memories of Maud, instead inventing new memories such as a childhood in Africa. After regaining her memory of her childhood, Florence converses with Robert about how he can also see the ghosts (his past war buddies who had died in battle).
It becomes apparent that those who are very lonely or haunted by the past can see spirits. Maud, who also sees Tom's ghost, explains that Tom is lonely and that he needs his family, which is why she arranged for Florence to come to the school in the first place.
Maud poisons herself and Florence, intending for their ghosts to join Tom. Maud tells Florence that the lonely children at school are starting to see Tom. Florence tells Tom that her soul will not be at rest if she dies now but that she will always be with him. Tom then helps Florence by bringing her medicine to throw up the poison but no vomit is ever seen.
The following scene shows Florence walking throughout the building, about to leave the premises with Robert. She passes by several adults on her way out but none notice her, even when she walks directly in front of them. One man even spoke badly of her after she passed right in front of him. The only people to acknowledge her presence are the redheaded boy and Robert.
Florence tells Robert to tell the driver to pull up to the end of the drive since she enjoys the long walk. Robert stands there to watch her walk away and exchange "hellos" with several children (but not with the adult amongst them), knowing that she cannot leave with him.