As the twelfth series of "Father Brown" unfolds, Father Brown (portrayed by Mark Williams) finds himself caught up in a dramatic battle reenactment, ultimately "falling" in combat. The event promises to keep the spirit of Kembleford alive and kicking. Frank Derbyshire (Nicholas Burns) mentions to Tyrone Higgs (Richard David-Caine) in a moment of distraction that he had forgotten the codpieces, a crucial part of their historical attire. Soon after, Walter Sullivan (Denis Lawson) arrives, baffling his son, Chief Inspector Edgar Sullivan (Tom Chambers), with his apparent confusion. Nearby, Sergeant Goodfellow (John Burton) continues to oversee the reenactment with fervor.
Walter is introduced to Isabel Devine (Claudie Blakley) amidst the festivities, and as they settle down for refreshments, Brenda Palmer (Ruby-May Martinwood) engages in a heated exchange with Tyrone over her exclusion from the proceedings. Father Brown steps in to inform the gathering that Tyrone heads the Kembleford Historical Society, but Brenda’s grievances persist, leading her to storm off in frustration. Tyrone recounts that the Battle of Kembleford was a conflict rooted in noble rivalry.
With the rehearsal underway, Father Brown joins Frank and Cecily Derbyshire (Melissa Johns). Frank reveals his reluctance to assist due to Tyrone’s reluctance to share the spotlight or tolerate challenges, sidelining him strategically. Cecily mentions their upcoming interview for a potential adoption, just as Dr. Alexander Manning (Luke Allen-Gale) arrives, hailed by Tyrone as Kembleford’s foremost living historian. Manning boasts of his discovery of the Alfred Ring, currently on display at the Royal History Museum. Frank reminisces with Brown about their school days, where they pushed each other to excel. Frank critiques the duel’s choreography, leading to a friendly sparring match with Tyrone, which Frank ultimately wins.
Next, Dr. Manning captivates the audience with tales of the ring, explaining how Godfellow lost it in a card game, along with Belside Hall. Walter and Isabel engage in conversation, the former inquisitive about her role as a secretary. Meanwhile, Father Brown discreetly confronts Frank about his earlier provocation of Tyrone. Frank shares that Manning’s promising future at Oxford was jeopardized when he was caught cheating on his High School Certificate exams. Walter’s pride is wounded when he learns that Father Brown might outperform Edgar as a detective, prompting him to rebuke Frank for tarnishing the Sullivan name.
Cecily’s temper flares when Frank casually mentions a letter from the adoption agency, which he had concealed. Later, in a romantic waltz, Father Brown and Brenda share a dance, setting a serene backdrop to the impending turmoil. Brenda, driven by curiosity, sneaks back and toys with the swords, only to discover Frank stabbed to death. Father Brown and Sullivan rush to the scene, Sullivan and Goodfellow noting the absence of a sword. Sullivan attempts to sideline Father Brown, who however, spots peculiar inscriptions nearby. Finding something crucial, Father Brown excuses himself.
Walter reprimands Edgar for allowing the priest to take charge, while Cecily’s heart sinks upon learning from Edgar about their missed adoption agency appointment, due to Frank’s secrecy about the letter.Frank clung tenaciously to the aspiration of fathering a child with his beloved. Meanwhile, Goodfellow unearthed the elusive sword hidden away in the dimly lit scullery. Cecily found herself ensnared by the police's suspicion over Frank's murder. At the precinct, she confided in Father Brown, protesting her innocence with earnest eyes. She recounted how her husband had recently made groundbreaking discoveries in his studies, necessitating a visit to Belside Hall for further investigation. Frank harbored two fervent dreams: to build a family and to etch his name in history as an esteemed historian. Seeking a second opinion, he had entrusted his research papers to Manning, who had feigned interest in discussing some newly uncovered historical tidbits about Frank. This revelation left Frank understandably apprehensive.
Father Brown, sensing a conspiracy to implicate Cecily, resolved to interrogate Manning. He delegated Brenda and Devine to delve into the pertinent materials. However, Walter warned Father Brown to steer clear of the matter. Inside the serene confines of the church, Brown confronted Manning, who insisted that Frank's historical research contained nothing remarkable, merely a curious tidbit that might have piqued Frank's interest. When pressed, Manning conceded to having cheated on his certification exams decades past, but emphasized that Frank's action, though responsible, had actually paved the way for his career success.
Walter offered his assistance during the reenactment of the events, while Brown and Brenda decided to scrutinize Frank's abode. Brenda griped about being sidelined from the investigative action. Edgar Sullivan cautiously requested Goodfellow to downplay any mentions of Father Brown should his father inquire. Walter's suspicion heightened as he caught Devine snooping around, which Edgar dismissed as mere costume checks. Brenda, in her quest for enlightenment, stumbled upon the tale of Joan of Arc.
Father Brown unearthed a forgotten record of the Battle of Kembleford that Frank had discovered in an antique archive in Cirencester just a month prior. He suspected that Frank had shared this with Tyrone Higgs and Manning. A servant revealed that Lady Catherine Feathergill had secretly slipped away with a priest before the dramatic breach of Belside Hall's doors, mentioning her intent to travel to Lancaster. Intriguingly, no record existed of Catherine's remains being found at Belside Hall. Brown also uncovered coded letters etched onto the armory walls, leading him to believe Frank was attempting to decipher them, possibly with assistance.
Walter Sullivan learned of Devine's unauthorized sleuthing, which Edgar tried to hush up amidst a flurry of anxious whispers. Devine's interference did not sit well with Walter, who warned of professional ruin. Cecily confessed to Father Brown her role in aiding Frank with the code, which he had stumbled upon at Belside Hall, inspired by Manning's book. She speculated that Frank might have already cracked the code, leaving her contemplative about confessing her involvement, her future looking bleak.
Father Brown then revealed to Isabel that the Feathergills had named their armory Lancaster, hinting that Frank had indeed solved the code, and they too must decipher it to uncover the truth.
Clad in gleaming armor, Brenda stands poised as Edgar approaches to converse with Devine, at which moment she subtly passes a crucial item to Father Brown, unnoticed by all but the observant. Edgar urges Devine to step away from the case, unbeknownst to him the gears of intrigue are already in motion. Father Brown seeks out Tyrone, his keen eye detecting that the fabric in his possession matches the very outfit Tyrone wears.
Tyrone reveals he had returned the previous night to try on his costume, encountering Frank on his way to the armory. A conversation ensues, sparked by Frank's intention to propose a vote of no confidence against Tyrone, a revelation that Frank dismissed with the remark of attending to "real history." Tyrone swears his innocence in Frank's demise, before joining the fray outdoors. There, he encounters Brenda, poised to intervene, and Father Brown pleads Tyrone's case for allowing Brenda to participate in the war.
Amidst the battle's chaos, Father Brown abruptly departs, prompting Devine to give chase. Brown confides in Devine that the code they deciphered is a Caesar Cipher, likely instructions from Lady Feathergill to her spouse. They uncover a clandestine doorway beneath their feet, leading them down a hidden passage. Tyrone informs Edgar of their dire need for Lady Feathergill to conclude their quest.
Reaching a hidden chamber, Devine discovers a button embedded in the floor, while Father Brown ponders whether they've stumbled upon St. Mary's crypt. Inside, he unearths the tomb of Prior Baskerville and a message from Lady Catherine, followed by a letter hidden within the wall, addressed to her lord.
Just then, Dr. Manning arrives brandishing a sword, announcing the resumption of their interrupted conflict. Father Brown queries the version of history Manning intends to uphold, suggesting that his truth is imposed by academic hubris. He reveals Frank's discovery exposed Manning as a charlatan, necessitating Frank's silencing. Brown accuses Manning of planting the button Devine found last night and of murdering Frank after the latter threatened to expose him. Furthermore, Manning confesses to framing Cecily, having witnessed her argument with Frank earlier.
With nothing left but to confront Manning, Father Brown vanquishes him using nothing more than his umbrella, assisted by Isabel's decisive blow. Walter and Edgar arrive at the crucial moment, piecing together the events. Cecily is freed from captivity, and Goodfellow produces Frank's ring from Manning's possession, a testament to Frank's legacy. Cecily reveals she carries Frank's other wish within her—she is pregnant.
The reenactment resumes, this time with Lady Feathergill escaping her fate. Tyrone, filled with remorse, apologizes to Cecily, acknowledging Frank's wisdom. Walter advises Edgar to depart, warning that his fiancée must cease her collaborations with the meddlesome priest if he is to bless their union. Thus, the tale unfolds, each thread interwoven with intrigue, passion, and the relentless pursuit of truth.
