As the curtains rise on another chapter of Father Brown's adventures, the clergyman, portrayed by Mark Williams, receives a missive from Sir Benedict Gellert (Christien Anholt). The letter bears tidings of Benedict's daughter, Marianne Gellert (Zoe Brough), who has been haunted by grim visions of spectral entities. Benedict's wife is consumed by fear, whispering tales of the supernatural—perhaps even demonic—origins. Desperate for guidance, they seek the aid of Father Brown.
Together with Brenda (Ruby-May Martinwood) and Isabel Devine (Claudie Blakley), Father Brown awaits transportation to the enigmatic castle. Shortly, Fenwick (Simon Day), the gamekeeper, arrives with a carriage, and Medusa steers them towards their destination. Sir Benedict and Dr. Marcellus Lasden (Clarence Smith) welcome them at the castle gates.
Upon entering, Father Brown offers congratulations on the recent wedding, but Benedict admits he scarcely recognizes his daughter. Marianne had suffered another episode the previous night, found wandering atop a cliff, as if possessed. She claims to see a cloaked apparition gliding through the castle corridors.
Lilith Gellert (Phoebe Pryce), Marianne's former tutor and now her spouse, disputes theories involving diet or poisoning. Fenwick attempts to intercept Benedict with a warning, but the latter brushes it aside. Lilith confides that she once caught Marianne and her friends reciting from a Satanic tome in the woods; she confiscated and incinerated it. Dr. Lasden, skeptical of Father Brown's presence, insists Marianne's plight stems from sources less sinister than the Devil. Father Brown concurs, suggesting the symptoms might be less satanic in origin.
Benedict directs Father Brown to Marianne's whereabouts. Despite instructions to await in the drawing room, Brenda and Devine wander through the castle. Brown engages Marianne in conversation about baptism and the beings she perceives. Marianne downplays the Satanic chanting as a mere game following a party, yet wonders if they inadvertently summoned darkness. She had been offered a spot at an art school in Paris, but her father put the brakes on her dreams amidst the turmoil. Marianne suggests her sketches were a product of trance-like states. Benedict cuts the conversation short, instructing Marianne to secure her chambers for the night.
Upon returning, Father Brown eavesdrops on Dr. Lasden professing a belief in the devilish influence. The doctor decides to stay another night, as do Brenda and Isabel. Father Brown joins Benedict for a libation, where Benedict reveals Marianne's lack of control during trance states. In the dead of night, Isabel startles Brenda during a bathroom quest.
The morning dawns with a grim discovery: Isabel awakens to find a lifeless body beside her. Benedict lies stabbed, and Fenwick accuses Isabel of foul play. Sergeant Goodfellow (John Burton) checks on Father Brown's wellbeing while Brenda and Edgar Sullivan (Tom Chambers) console Devine elsewhere.
Isabel insists that she drifted off in the guest room only to awaken in the tower, her memories of the interim a blank slate. Dr. Lasden casts a skeptical eye on her claim of temporary amnesia, finding it conveniently timed. Just then, Marianne bursts in, her accusations against Isabel ringing loud and clear: she accuses her of murdering her father. As an aside, she postulates that perhaps a horned creature was to blame, but Dr. Lasden swiftly corrects her, asserting that the perpetrator was all too human and will face the noose for their deeds. Father Brown turns to Lilith, inquiring if there was any particular reason Benedict was resting in the tower. Lilith concedes that Marianne, in her trance states, can display aggression, though she herself was confined to her room under lock and key the previous night.
Approaching Marianne, Father Brown queries how she managed to escape her quarters that morning. Marianne reveals she procured a spare key from the servants' quarters. He then notes that Benedict's sketch appears to have borne the brunt of someone's wrath. Marianne admits to harboring occasional resentment towards her father for restricting her from leaving their confines, yet hastily emphasizes that her father meant the world to her.
Dr. Lasden informs the police of his encounter with Mrs. Devine near the attic stairs; she explains away her presence by saying she was in search of the bathroom and lost her way. Brenda recalls seeing Isabel near her door around the same timeframe. Fenwick chips in, mentioning Benedict's instruction for him to take his tea away from the maid. Father Brown proposes that Isabel couldn't be the murderer, reasoning that she would have fled the scene had she been the culprit. In a surprising twist, Isabel urges Edgar to arrest her, though Goodfellow steps in to handle the matter for him.
Outside, Edgar confides in Father Brown that one among them must surely be the killer. Inside, Marianne consigns Benedict's sketch to the flames in the fireplace. Father Brown and Edgar converse with Fenwick outdoors, the priest probing into the reason behind Fenwick's urgency to speak with Benedict. Fenwick reveals that Benedict intended to allow developers to build on part of the castle land, which happens to be an ancient burial ground. Fenwick fears that disturbing these grounds could awaken the spirits of the deceased, but vehemently denies any intention of harming Benedict.
Returning indoors, Father Brown finds Marianne in the throes of another episode. Asked to rummage through Dr. Lasden's bag, he discovers something intriguing and slips it into his pocket undetected. Once Marianne regains composure, she relates that she saw no strange figures this time around.
Lilith informs Father Brown that he holds the freedom to wander anywhere within the castle's confines, provided he uncovers the mystery ailing Marianne. Brenda, startled by the slamming of a door, catches sight of something that sends shivers down her spine, only to receive an unexpected jolt from Fenwick shortly after. Edgar urgently informs Father Brown of his necessity to attend the station, as Devine's faintness and labored breathing necessitate immediate attention. Father Benedict reveals to Brenda a damning letter penned by Sir Benedict, terminating his illicit affair with the doctor.
Lilith confides in Dr. Lasden, expressing her belief that he has exhausted all possible avenues in treating Marianne. Father Brown confronts Lilith, revealing his witness to her slipping a note into the doctor's satchel. She concedes her attempt to persuade Benedict to end the affair, acknowledging his relentless nature and her marriage to him as a mere facade for societal approval. Yet, Lilith steadfastly maintains her innocence in Benedict's demise.
Dr. Lasden informs Marianne of his intention to return to London, acknowledging her plight as possibly psychological yet acknowledging his own sense of failure. Marianne pleads with him to stay, highlighting his constant presence as a comforting fixture. Lasden, grief-stricken by the loss of his best friend Benedict, finds his departure too painful and bids her farewell. Outside, Father Brown approaches Lasden, offering an apology and handing over the letter. He inquiries about a mark on Benedict's cheek, suggesting a blow from someone adorned with a ring. Lasden's recount of their confrontation paints a vivid picture, with Brown querying whether Lasden had struck Benedict himself. A flashback ensues, illuminating the tense encounter. Lasden concedes, "I could not compete with the invisible presence in the heavens."
Brenda, Marianne, and Lilith stumble upon bloodstains on a nearby rock, prompting Brown's assessment that the rock had served as a grim weapon. Brenda remarks on Fenwick's prolonged absence, sparking Marianne's anxiety over their shared fate should they remain. Edgar endeavors to soothe Devine's distress, while Goodfellow reports the absence of incriminating details from background checks. Father Brown encourages Marianne to cling to hope, amidst Brenda's revelation that Lilith has summoned the authorities over the discovered blood.
Brown discovers a sketch depicting Marianne's haunting visions, leading him and Brenda on a quest through religious and folklore tomes. He speculates a connection to Cernunnos, the god of nature, weaving a tapestry of myth and mystery around their investigation.
Brown asserts that pagans employed hallucinogenic plants, yet the method through which Marianne consumed them remains a mystery. Brenda sets off to inquire of Marianne whether these plants grow naturally on the ground, her footsteps echoing softly until they are interrupted by the sharp sound of glass shattering, followed instantly by an unseen force grasping her from behind. Meanwhile, Brown notices a book lying open, its side smeared with a dark thumbprint that he speculates could have originated from the soot-laden fireplace nearby.
Edgar extracts Devine from her cell to conduct an interview, during which she recounts waking up in the tower with a minor headache. Edgar's keen observation catches sight of scratches adorning her legs. Elsewhere, Father pursues a shadowy figure cloaked in darkness, shouting that he has identified their guise. He trails the cloaked entity through an intricate maze outdoors, until a sudden blow from behind fells him unconscious.
Awakening in the rear of a carriage alongside Brenda and Fenwick, Brown finds Lilith explaining that the green cloak is a relic of her ancestors' tribal attire. She delves into tales of Cernunnos with Brown, revealing her ties to the ancient, nearly extinguished religion of the Stacga, which faced near annihilation at the hands of Christians. Her mission is to summon Cernunnos from beyond, a task that requires his adversary's demise at the hands of a tribal chieftain on sacred soil. Lilith confessed to slaying Benedict to prevent him from allowing developers to defile the sacred land. She believes she is poised to perform the ritual that will revive Cernunnos, orchestrating their deaths to resemble mere accidents.
Marianne arrives on the scene, almost causing Lilith to plummet to her doom. Father Brown acts swiftly to save her, just as Edgar and Goodfellow arrive to apprehend Lilith. She warns Brown that this is merely a prelude. Edgar expresses gratitude to Brown for unraveling the mystery. Dr. Lasden, emotional, tells Marianne he regrets ever leaving her side, revealing that she is the daughter of the man he deeply cherished. Marianne confides in Brown her intention to relocate with Lasden to London, ultimately pursuing her passion at an art school. As she departs with Lasden, she gifts Brown a drawing of Benedict.
