The episode kicks off with Cyril Frogmore (Kenneth Collard), the Kembleford art instructor, and Gaynor Garfield (Ingrid Oliver) engaged in a spirited discussion over a blueprint. As Father Brown (Mark Williams) and Mrs. Devine (Claudie Berkley) step into the store, Cyril dismisses Meryl Plunkett’s (Lydia Larson) charming trinkets as insufficient to grant her artist status. Gaynor counters, asserting that arts and crafts are intertwined. Enthused, Mrs. Devine signs up for the "Life Drawing Competition," only to be informed by Gaynor of the model's unfortunate bout with measles. Undeterred, Mrs. Devine volunteers to find a substitute. Gaynor, holding aloft the coveted trophy, reveals that the winner's portrait might grace the pages of the Gazette, much to Cyril's caution against raising false hopes. Father Brown commends Mrs. Devine for her commendable community efforts.
Gaynor's mood dampens upon discovering one of her pieces featured in a journal, prompting her to label Meryl’s endorser an "imbecile." Seeking a replacement judge for the affected individual, Gaynor enlists Father Brown. Marmaduke Snell (Edward Bennett) arrives, eager to purchase Gaynor’s work and insists on participating in the Life Drawing, despite Cyril's stipulation for locals only. Gaynor, bending the rules, approves his registration. Snell, sarcastic as ever, jokes about the affordability of his art at the event.
Outside, as Snell gathers his creations, Sister Boniface (Lorna Watson) loses control of Agatha, her bicycle, narrowly missing him. Labeling her a "crazy penguin," Snell threatens legal action, though Father Brown assures him it was a mere accident. Unrepentant, Snell, after expressing his reluctance to forgive, helps himself to Sister Boniface’s wine.
Cyril seeks Father Brown's assurance of impartiality in the competition. Father Brown reassures him of his fair judgment. Snell purchases a piece from Meryl, who excitedly proclaims it her first sale. However, Snell cruelly informs her it will also be her last, citing a lack of "artistic merit," and plans to use it to clean his muddied shoes. Devastated, she rushes off, prompting Father Brown to rebuke Snell.
Later, Father Brown informs Mrs. Devine and Brenda Palmer (Ruby-May Martinwood) of his judging role. Brenda expresses doubts, to which Father Brown reassures her, hinting at Cyril's displeasure should he lose. As patrons sketch the male model, Chief Inspector Sullivan (Tom Chambers), Gaynor announces Meryl's absence as the bell rings. Sullivan praises Snell’s piece, and Snell, thanking Father Brown for his victory, earns Gaynor's ire with his smug demeanor. He dismisses her concerns before guzzling wine and suddenly foaming at the mouth, collapsing to the floor. Sergeant Goodfellow (John Burton), checking for a pulse, pronounces Snell deceased. Detecting an almond scent, Father Brown suspects cyanide poisoning in the wine.
Sullivan interrogates Sister Boniface concerning the wine from St. Agnes' vineyard that Snell had consumed, his tone laden with suspicion. She reassures him, her voice firm, that the wine itself was untainted, as she had personally sealed the bottle. She posits that the cyanide might have been on the bottle opener instead. His response is swift, revealing that the cork had not been fully secured. of Snell's threat to lodge a complaint against her, an accusation that leaves her unperturbed. "That hardly justifies murder," she retorts calmly.
Later, Sullivan breaks the news to Sister Boniface and Father Brown: St. Agnes' wine operations have been temporarily halted. Father Brown, in turn, relates to Sister Boniface, Divine, and Brenda how Snell had insulted Meryl's artwork, adding an intriguing detail—a red substance stained the banknotes found in Snell's pocket.
Arriving at the venue, Father Brown and Sister Boniface find the door locked tight. With a deft hand, Father Brown picks the lock, and inside, Sister Boniface discovers seeds embedded in the soil clinging to Snell's shoes. Confessing her discontent at St. Agnes, she considers Father Brown's suggestion of requesting a transfer but rejects it, reminding him of her years of dedication. He mentions metal polish casually, prompting her to reveal that some varieties contain potassium cyanide.
Outside, Gaynor persists in convincing Sullivan to attend the life drawing session, dangling Snell's portrait of him as bait. Meanwhile, Sister Boniface and Father Brown observe Gaynor surreptitiously drop two tablets into her drink, which she blithely describes as a "pick-me-up." When questioned about her altercation with Snell, Gaynor claims Snell coerced her into selling her artwork, her voice tinged with resentment. Sister Boniface offers solace, assuring her that there's no longer a need to sell, but Gaynor vows, "You'll regret this for the rest of your life."
Sister Boniface collects a seed for her investigation as they step outside, and upon reaching Snell's apartment, they find the door slightly ajar. Inside, a notepad reveals indented writing. Using a pencil, Sister Boniface shades in the letters to uncover the message: "Dearest agent, unavailable until the 26th. Cancel RCA Angela. Renting in dreary Kembleford to collect something of mine." Father Brown ponders what Snell intended to retrieve just as Sullivan and Goodfellow arrive, prompting the pair to slip out the back entrance. Sullivan informs them that cyanide has indeed been detected in the wine, his voice laden with gravity.Sister Boniface has been placed under arrest on suspicion of murder, amidst a cover-up orchestrated by Brenda, Father Brown, and Devine. Goodfellow draws attention to the fact that Snell's wine bottle was uniquely contaminated with cyanide, setting it apart from the others.
Later, Father Brown pays Sister Boniface a visit, enlightening her that RCA is an acronym for the Royal College of Art. With a pledge to uncover the genuine perpetrator, he bids her farewell.
Turning his attention to Cyril, Father Brown inquires whether he was a fellow student of Snell's at the Royal College of Art. Cyril responds with skepticism. Upon inspecting Cyril's wallet, Father Brown notices red paint stains on the banknotes, mirroring those found on Snell's money. Confronted, Cyril confesses to bribing Snell to keep his non-graduation from RCA under wraps. He further reveals that Snell and Molly had planned their honeymoon shortly after. Father Brown hints that Sullivan might refund his money if Cyril comes clean about his dealings with Snell.
Father Brown then requests Goodfellow to lend him the evidence from Snell's murder case, intending for Sister Boniface to scrutinize it. In the meantime, Devine volunteers to go to jail, stepping into Sister Boniface's shoes behind bars.
Elsewhere, Sullivan poses as a model for the art fair, with Meryl voicing doubts about her capability. Cyril encourages her not to let fear dictate her actions, urging her to join the other patrons. Gaynor sets the timer, marking the start of the event. Sullivan inquires about Devine's whereabouts, to which Brenda replies that Devine is auditioning for a role at the Players Theatre. Suspecting foul play, Sullivan abruptly leaves the art fair, abandoning his role as a model without a second thought.Examining a wedding band adorned with an intricate encryption – "M&M May 29, 1954," Father Brown deciphered the hidden message. Sister Boniface, meanwhile, meticulously inspected a cork, noticing microscopic marks indicating it had been pierced by a needle, suggesting the perpetrator had injected something into it. Sharing his observations with her, Father Brown mentioned Cyril's claim that Snell and his bride had been planning a honeymoon. Sister Boniface's curiosity piqued, she inquired about the delay in their trip.
Inspector Sullivan approached Goodfellow, inquiring about Mrs. Devine's whereabouts. Goodfellow replied he hadn't seen her but had heard she had an audition.Sullivan then went to check on Sister Boniface, only to find Devine masquerading as her. As Gaynor greeted Father Brown upon his arrival for his judging duties, he selected Meryl as the winner, much to her delight. Meryl, now convinced, decided to set up her stall, and Mr. Frogmore offered his assistance.
Father Brown requested a private conversation with Gaynor. Once they were alone, he addressed her as Molly and acknowledged her emotional reaction upon seeing her artwork featured in the journal. He surmised she had realized it could lead Mr. Snell to her. Father Brown revealed something she had overlooked during her search of Snell's cottage before mentioning his true purpose for visiting.
Molly confessed that her marriage had taken place mere days before her arrival in Kembleford. She admitted to meeting Snell two years prior, embarking on a whirlwind romance. However, on their wedding day, Molly realized she had made a grave mistake. Snell wanted her to stay home and start a family, prioritizing domestic life over her artistic pursuits. Unable to reconcile with this, Molly left him on their wedding day, taking some of his savings.
Snell tracked her down and manipulated her until they were alone. He threatened to expose her theft to the police unless she returned with him, reminding her of the vows she had taken.
Snell requested Molly's presence at Chipping Way once the fair had concluded. Father Brown harbored suspicions that Molly had contaminated his wine out of desperation to escape her entanglement with Snell. He speculated that, being diabetic, she might utilize insulin needles for a different purpose – one of these needles, he conjectured, could have been employed to draw up Miss Plunkett's metal polish, subsequently injected into Snell's wine bottle through the cork. Molly acknowledged without remorse that she was not grief-stricken by Snell's demise.
As Frogmore and Meryl entered the scene, Molly confessed to them that her liberation from Snell was imperative. Father Brown exhorted Molly to repent and confess her sins, warning that failure to do so would exact a severe toll, potentially costing Sister Boniface her life. Molly, who had finally reconnected with her family, was reluctant to leave. Frogmore offered to oversee her shop during her absence. Meanwhile, Devine narrowly escaped his cell just as Sullivan returned, alerting him to something amiss. Father Brown's timely arrival informed Sullivan that it was time to free Sister Boniface.
Molly stepped forward, acknowledging that it was indeed time for her confession. Sister Boniface retreated to the confessional, where she imparted her confessions to Father Brown. She confessed that she had felt an unparalleled sense of exhilaration over the past few days, a sentiment she attributed to Father Brown's influence. He assured her that her heightened spirits stemmed from embracing her innate talents. Having taken his advice, Sister Boniface had requested a transfer from St. Agnes'. Father Brown encouraged her to harness her exceptional gifts for the greater good.
Outside, Sister Boniface announced to the gathering that a fresh chapter awaited her at St. Vincent's. She posed for a picture with Father Brown, who remarked that they made a formidable team. Sister Boniface believed she had discovered a new purpose in life, thanks to him. As the episode drew to a close, she departed, carrying with her a renewed sense of calling.
