As the curtains rise on this latest chapter of Father Brown, Violet Goodfellow (Naomi Radcliffe), her voice tinged with enthusiasm, addresses the gathering at the sewing circle. She proposes an exhibition of their meticulous crafts in the heart of the village, a suggestion that falls flat on Catherine Glover's (Carlyss Peer) indifferent ears. Catherine scoffs, invoking the legendary Kembleford Tapestry as a benchmark of true challenge, suggesting they present it at St. Mary’s for all to admire. Judith Skelton (Felicity Montagu) heatedly disputes this notion, her voice rising in defense. Catherine, undeterred, wonders aloud if they’ve lost their sense of purpose, asserting that this endeavor could prove they are more than mere idle housewives. Violet counters, her tone laced with frustration, that she lacks the time for such an ambitious project. Catherine, seizing the moment, accuses Violet of losing her visionary edge and declares herself the new leader of the sewing circle, promptly calling for a vote.
Later, in the confines of their car, Violet confides in Sergeant Daniel Goodfellow (John Burton), her voice tinged with resignation, that the group has opted for a new direction. Catherine’s smirk as she strides away paints a picture of victory. In a quiet corner, Violet unveils her work to Father Brown (Mark Williams) and Brenda Palmer (Ruby-May Martinwood), explaining that each panel they’ve crafted chronicles a significant era of Kembleford’s past. When pieced together and framed by an elegant border, this tapestry will be installed at St. Mary’s for a grand unveiling on Sunday. Judith, renowned for her exceptional needlework skills, offers sage advice nearby. Violet admits, with a hint of resentment, that Catherine Glover is the driving force behind this venture. Catherine abruptly interrupts, demanding to know why the pamphlet credits Violet, not her, as the head of the sewing circle, accusing Violet of jealousy over her leadership.
Father Brown and Brenda take Catherine aside for a private conversation, where she reveals she’s entrusted someone else with the honor of the unveiling. Their discussion is gently interrupted by Canon Fox (Roger May), who greets them warmly. Elsewhere, Isabel Devine (Claudie Blakley) shares her concerns with Edgar Sullivan (Tom Chambers), worrying that his parents might not attend their wedding, adding a layer of poignancy to the unfolding narrative.
Edgar concedes that her companionship and endeavors with Father Brown are the crux of the matter. Walter, however, remains steadfast in his refusal to lend his approval until Edgar insists that she ceases her collaboration with the clergyman. Catherine and Canon elucidate that they have orchestrated for the media and the mayor to be in attendance at the upcoming unveiling ceremony. Meanwhile, Violet and Judith engage in a discussion about Catherine's decision to bypass Father Brown. Martha reminds Judith of her earlier vote of confidence in Catherine.
In a private conversation, Father Brown and Canon Fox delve into matters of grave importance. Father Brown has heard whispers that Bishop Reynard is contemplating retirement, to which Canon confesses, revealing his desperate need for the unveiling to proceed without a hitch. Elsewhere, Father Brown lends a helping hand to Goodfellow and William Fitzgibbon, alias Robert Wilfort. Brenda comments to Catherine, hinting that her work might actually outshine Judith's. An awkward moment ensues when William accidentally drops his comb. Father Brown attempts to engage him in conversation and discovers that William is an agnostic and a professor of mathematics at Gloucester University. William extends an invitation to Father Brown to join him and Daniel at their weekly gatherings.
Judith inquiries about the border issue with Catherine, while Martha laments that Judith's condition seems to be deteriorating. Violet intervenes, pleading for compassion towards Judith. Brenda steps outside and catches Catherine in a hushed conversation with William. Catherine expresses her frustration, insisting that their relationship must remain concealed. William cautions her about the potential repercussions if their affair were to come to light. Overwhelmed by guilt, Catherine resolves to confess everything. Brenda hastens to inform Father Brown, just as Isabel joins their group, only to dart away shortly thereafter. She reunites with Violet and the others just as they stumble upon Catherine's lifeless body.
At the crime scene, Father Brown finds it peculiar that Catherine's panel is the sole one defaced. Inside her purse, they discover a copy of The Times Crosswords and a poem, the latter not penned in her handwriting. Brenda suggests the possibility of a secret lover. Edgar interrogates William at the police station, to which he maintains his innocence, insisting that he and Catherine were merely friends and that he would never harm her. Amidst his statements, William utters something about treason. Martha confides in Father Brown her fears that William might crack under pressure. Father Brown assures her that he will visit William the following day to offer him solace.
In the privacy of a secluded corner, Daniel Goodfellow confides in Father Brown, revealing that his visits to William have been forbidden. Daniel recounts how William, who harbored a deep aversion to violence and refrained from bearing arms during wartime, instead served in the Communications Office. With earnest pleading, Daniel urges Father Brown to uncover the true perpetrator of the crime.
Father Brown then engages Brenda and Isabel in a conversation punctuated by his concerns. He remarks on the poem's lack of romanticism and puzzles over William's motivation to vandalize the tapestry panel. Isabel attempts to distance herself from the matter, eventually joining the sewing circle. Later, Brown discusses with Catherine her replacement and the appearance of her details on the leaflet, prompting her to gripe about Judith's absence. Sensing the need to delve deeper, Father Brown decides to pay Judith a visit.
Daniel strives to convince Edgar of his innocence, pointing out a possible murder weapon inscribed with William's initials. Meanwhile, during Father Brown's visit, Judith's great-niece discreetly takes possession of his belongings. Brown reassures Judith of his concern for her well-being, but she expresses her desire to withdraw from the project, deeming it doomed. Observing her movements, Father Brown suspects she might be suffering from arthritis and speculates that Judith has been grooming Alicia as her assistant.
Alicia attends the same school where Catherine once taught. Catherine had caught Alicia aiding Judith in a competition cheat and threatened exposure, forcing Judith to support Catherine's takeover bid at the Annual General Meeting. When confronted, Judith vehemently denies any involvement in Catherine's demise, instead hinting that Martha might have been aware of Catherine and William's relationship.
Violet later informs Isabel, Brenda, and Brown that William has been formally indicted, with the murder weapon purportedly one of his compasses. Father Brown finds it peculiar that William would use such a distinctive item. Brenda counters that Martha, too, could have accessed his compasses. Isabel excuses herself from the conversation. Edgar informs Daniel that he has extracted all necessary information from William, asserting that no one can interview him while he's in charge. Isabel reveals to Violet her lunch plans with Edgar, only to be warned that her job will always take precedence.
Canon Fox storms in, demanding to know why he was kept in the dark about the murder. Judith arrives, insisting that the tapestry project will be completed, and reveals that she has something crucial to share with everyone. As Isabel bumps into Brenda and Brown, she mentions her intention to visit Catherine's place to retrieve the original plans. Upon arrival, they find the door ajar and the house in disarray. Martha descends the stairs, revealing that she discovered poems sent to William, hidden in his tie drawer, and recognized Catherine's handwriting. She firmly believes William incapable of harming her and denies any involvement in her death. Martha scorns Catherine as unpredictable and incapable of caring for William properly, asserting that she found her own means of revenge. She admits to altering the name on the leaflet but insists she only came to search for evidence. Martha presents Brown with the poems she found and shows him William's comb, which Brown notices has a slightly different hue than the one missing.
He declares it to be a Cardan grille, intriguing Brenda enough for Father Brown to demonstrate its mechanics. William and Catherine, it seems, cleverly concealed their clandestine communications within the poems. While Brown delves into deciphering these conversations, Martha and Isabel inspect the tapestry with keen eyes. Violet sighs, lamenting the loss of the final insignia, while Isabel, unable to locate Catherine's designs, suggests she could still play a part in the unveiling ceremony. Brenda rouses Brown from his slumber, eager to know if he's uncovered any new insights. Brown reveals that the letters hint at William and Catherine's collaborative efforts during wartime, shrouded in secrecy so profound that discussing them would have breached the Official Secrets Act. They ponder the mystery behind Catherine's desire to reveal all this now. Isabel informs Father Brown of their need for his presence at St. Mary's, where the unveiling has been renamed in Catherine's honor.
Eager to understand more, Father Brown seeks an audience with William, and Daniel, defying Edgar's orders, grants him permission. Meanwhile, Isabel and Brenda keep Fox company, with Isabel subtly attempting to conceal the tapestry's disappearance. In his conversation with William, Brown delves into the decryption of Nazi radio codes, suggesting that Catherine masterminded the translation of German intercepts. William concurs, hailing Catherine's genius but acknowledging her troubled mind, often consumed by obsessive thoughts.
Brown pleads for William's assistance, emphasizing that a killer remains at large. William recounts how Catherine stumbled upon a report of a Luftwaffe air strike on a naval vessel, a moment she recognized from her translations. Haunted by the notion that she might have made an error, Catherine became determined to confess her role at the unveiling. She even embroidered the ship's emblem onto her tapestry, symbolizing the aircraft carrier HMS Lionheart. Assuring her that no one would fault her, William notes Father Brown's skepticism. Brenda then breaks the news to Brown: the tapestry has been stolen. Brown is convinced that the same perpetrator who killed Catherine is behind this theft, adding a layer of urgency to their quest for answers.
He discovers Judith in the midst of preparing to incinerate the tapestry, her determination etched on her face. Brown is acutely aware that her steadfast refusal to celebrate Catherine's achievements stems from a deep-seated grief over her son. The image of her son James flashes through his mind, and he suspects that James might have served aboard the HMS Lionheart. Judith concedes that she had opened up to Catherine about her son's connection to the ship, a revelation that seems to weigh heavily on her conscience. Catherine, haunted by doubts, acknowledges she might have overlooked the nuances in her translation. The conversation degenerates into a heated altercation, culminating in Judith lashing out at her. With a mixture of defiance and resignation, Judith informs Brown that Catherine craved punishment, and so she granted her that wish.
Meanwhile, Isabel persists in her efforts to delay Canon Fox, while Father Brown engages Judith in a heartfelt dialogue. Judith, her voice tinged with regret, offers an apology. She clings to the belief that James's death was unnecessary and that Catherine could have intervened to save him. Father Brown gently reminds her that individuals like Catherine operated under extraordinary pressures. He encourages her to come clean with the authorities. Curiously, Judith inquiries about his sewing prowess, a sudden shift in topic that leaves Brown momentarily puzzled.
The revelation that the tapestry is missing sends shockwaves through the group, with Canon and the others expressing disbelief and concern. Miraculously, Father Brown reappears shortly thereafter, the tapestry safely in his possession. William's release from jail brings a modicum of relief, though Edgar's displeasure at Daniel's decision to allow William a visitor is palpable. He warns Daniel that the Special Branch expects a thorough report, and Daniel's apology falls on deaf ears. Edgar hints at potential disciplinary action against Goodfellow, leaving Daniel to ponder the gravity of the situation.
Daniel catches up with Violet, who reveals her reinstatement as the head of the sewing circle, oblivious to Daniel's own suspension. Isabel, determined to find Edgar, sets off on a quest to locate him. Canon vents his frustration to Father Brown, who has inadvertently made him appear foolish. Isabel locates Edgar and initiates a profound conversation, touching on his career aspirations and his father's influence. She confesses her love for Kembleford but asserts that her love for Edgar transcends geographical boundaries. She urges him to accept the promotion in London's Special Branch, suggesting they embark on a new chapter together in the bustling metropolis.
