The episode kicks off with a man paying his respects at Louisa Ross's (Bethan Leyshon) graveside, only to be startled by the fresh mound of earth. His heart pounding, he dashes to a nearby phone booth, eagerly conveying his disturbing discovery to an unseen recipient.
Meanwhile, at Saint Mary's Church, Brenda Palmer (Ruby-May Martinwood), Mrs. Devine (Claudie Blakely), and Father Brown (Mark Williams) are immersed in preparations for Louisa's wake when Janice Higgins (Lois Pearson), the hysterical housekeeper, bursts in, her voice filled with desperation. She pleads for their immediate assistance. Father Brown, joined by Brenda, hurries with her to Ross Manor, where they are met at the door by the awkward and unsettling Gilbert Gallamore (Nicholas Asbury). Mrs. Devine, though invited, chooses instead to visit the ailing Inspector Edgar Sullivan (Tom Chambers), prioritizing his welfare over curiosity.
Inside Ross Manor, Father Brown encounters a deeply depressed Bernard Ross (Nicholas Woodeson). The revelation about the fresh dirt at Louisa's grave sparks a fiery debate about the existence of vampires. Gilbert eggs Bernard on, using Louisa's peculiar behavior and the symptoms of her illness as evidence for their existence. Father Brown, no stranger to such claims, counters with the coroner's ruling on Louisa's mental state, seeking to dispel the supernatural fog. The argument escalates, spilling out into the rear garden where, inexplicably, the corpses of small rodents have begun to appear since Louisa's demise.
Enter Silas O'Hagan (Ray Fearon), the vampire hunter, whose timely arrival ignites a renewed sense of urgency to revisit Louisa's gravesite. Elsewhere, Mrs. Devine tends to Sullivan's failing health, while he, despite his condition, remains in charge, issuing orders to Sergeant Goodfellow (John Burton) over the phone. Goodfellow, perhaps sensing an opportunity to prove his mettle, seems a tad too gleeful about Sullivan's plight, his facial expressions betraying a hint of ambition.As daylight bathed Kembleford, the discourse on vampirism resumed at Louisa’s gravesite. Christine Kipley, alias Josie Lawrence, revealed her stance on the matter promptly, her identity as Kembleford’s Sunday school teacher and Bernard’s sister-in-law adding weight to her words. Upon hearing of Silas’ invitation to Louisa’s dinner, her initial instinct was to decline. Later, amidst a toast where Bernard poignantly recounted Louisa’s affliction—how it cruelly curbed her love for the outdoors, wildlife, and sunflowers in particular—the evening’s ambiance shifted.
Silas opened up about his vampire beliefs, producing an assortment of customized stakes as proof. Meanwhile, Christine gently scolded Gilbert for his excessive wine consumption when he offered to replenish everyone’s glasses. Moments later, as he departed to refresh his own, a piercing scream from Janice drew the gathering’s attention to the library, where Gilbert had, in his inebriated state, made inappropriate advances.
The morning after, Father Brown and Brenda’s return to the Manor unveiled a grim discovery: Gilbert’s lifeless body, bearing two puncture wounds on his neck and completely drained of blood. Father Brown struggled to rationalize the murder within natural terms, the supernatural seeming all too plausible. The case took a starker turn when Louisa’s necklace was found.
Upon learning of the murder, Sullivan, despite Devine’s disapproval, attempted to rise from his bed and dress. However, his aversion to sunlight, among other symptoms, hindered his efforts. Father Brown’s confirmation that Louisa had been buried with the necklace he retrieved prompted him and Goodfellow to forge a partnership in unraveling the mystery.
Their investigation began with Janice, who shared that Gilbert and Bernard had been history professors together at Oxford. Although Janice’s account of Gilbert’s drunken advances was distressing, it was her revelation of catching Christine rummaging through Louisa’s desk that truly stunned them.
With the newfound discovery as leverage, Father Brown coaxes Catherine into confiding in him about a harsh, incriminating letter that left her deeply distraught over Louisa's steadfast refusal to accept the church's assistance. Though its contents remain undisclosed, hints suggest a storm of emotions brewing within her, vowing not to let Bernard suffer the same misfortune. As their conversation draws to a close, Father Brown and Goodfellow cannot conceal their skepticism when they chance upon Silas, peddling mustard seeds and holly outside the manor gates. The encounter yields little of consequence, prompting Father Brown to seek out Bernard, who is en route to Louisa's gravesite.
Their conversation unearths that Louisa's mother succumbed to an illness beyond the doctors' comprehension, and Bernard reveals the existence of Louisa's diary, which he insists he has never perused. He firmly denies Father Brown's request to examine it.
Just as Father Brown, Brenda, and Mrs. Devine are enjoying their lunch, Goodfellow bursts in, initiating a lively exchange of theories about the bizarre nature of Gilbert's exsanguinated corpse. Mrs. Devine's abrupt departure to attend to a call from Sullivan leaves the table momentarily deserted. Upon her return, she shares the grim news of Sullivan's deteriorating condition and urges Goodfellow to accompany her on a visit.
Meanwhile, Father Brown engages Silas and Christine in an investigation of mysterious footprints outside the church. Silas speculates that Louisa might have attempted to sabotage the mass, thwarted only by the fact that vampires cannot breach the sanctity of a house of worship. He also relates overhearing a heated exchange between Bernard and Gilbert, touching upon Gilbert's relationship with Louisa. Goodfellow's visit to Sullivan serves as a stark reminder of a neglected fraud case in Wedlock that he had been tasked with investigating amidst the vampire-infused excitement.
Upon his return to the station and diving deep into the intricacies of the fraud case, Father Brown uncovers a startling revelation: both cases are intertwined with supernatural undertones. He promptly arranges a meeting with Bernard, intending not only to quiz him about the recent altercation but also to request a second glimpse into Louisa's diary. Gaining access once again to the intimate confessions of Louisa's heart, he discovers that Gilbert and Louisa forged a bond rooted in her illness. Gilbert had convinced her that she was cursed, a belief that left her embarrassed and prompted her to distance herself from the church.
As Father Brown shares these revelations with Brenda, a seemingly unremarkable call from a doctor interrupts their conversation briefly. Just as they resume their discussion, Brenda bursts in, urgency etched on her face, informing him that his presence is immediately required at the church.
At the church, they catch Bernard in the act of desecrating Louisa's grave, but they intervene just in time. Goodfellow arrives on the scene almost instantly, revealing Silas's true identity as the deceitful conman they had been searching for. It transpires that Silas is none other than the perpetrator of the fraud case that Sullivan had previously mentioned. The scar on Silas's forehead acts as the pivotal clue that connects him to the crime. Goodfellow also presents the post-mortem report, which confirms Father Brown's theory that Gilbert's puncture wounds were inflicted after his heart had ceased beating.
Father Brown elucidates Louisa's symptoms as stemming from a rare medical condition known as hereditary coproporphyria, which can also induce seizures, potentially explaining her mother's affliction as well. Throughout the unfolding of the evidence, Silas never denies his role as a fraud but steadfastly maintains his innocence in the murder. When Goodfellow attempts to apprehend him, Father Brown requests a brief, private conversation with Catherine.
It is during this intimate exchange, with the family photo on the wake pamphlet serving as a poignant reminder, that Father Brown deciphers the mystery surrounding the necklace. It transpires that Louisa's mother had bestowed identical chains upon both Catherine and Louisa. Ultimately, the veil is lifted, revealing Catherine as the perpetrator of the crime. The episode concludes with a moving mass and absolution for Louisa, attended by both Bernard and Sullivan, as they seek solace and closure.
