As the highly anticipated twelfth and final episode of "Father Brown" unfolds, Father Brown (portrayed by Mark Williams) and Brenda Palmer (Ruby-May Martinwood) confront Mrs. Devine (Claudie Blakley), querying her absence from the audition. Brenda, additionally aware of the newly arrived suitcases, joins the inquiry. Mrs. Devine reveals her intention to hand Father Brown a letter for days now and admits that post-wedding, they will relocate to London.
The narrative shifts to Mrs. Devine discussing her wedding preparations with Lady Felicia Montague (Nancy Carroll) and Irene Frost (Michelle Bonnard). Amidst these conversations, she expresses her dissatisfaction with the lack of vibrancy in the arrangements to Eddie (Barney Wilkinson), Brenda, and the gathering. Felicia, invoking the elegance of Audrey Hepburn, reassures Devine, offering to take charge of the hairstyling and floral arrangements. Eddie shares his thoughts on London with Brenda, professing his affection but bewilderment regarding his mother's desire to move.
Father Brown intervenes, acknowledging Devine's dissatisfaction and feeling overwhelmed by the planning. He presents her with a locket, encouraging her to have the wedding day she desires. Outside, Irene gently reminds Devine that the aesthetics are immaterial as long as she is united with the right partner, sharing her own unsuccessful engagement story. Chief Inspector Sullivan (Tom Chambers) arrives, presenting Isabel Devine with a London map to aid her navigation, amidst her excitement to leave her current surroundings.
Walter Sullivan (Denis Lawson) and his wife, Grace (Cherie Lunghi), soon arrive, with Walter suggesting that Father Brown might revert to his true duties once Mrs. Devine departs. Sullivan informs his father that Sergeant Goodfellow (John Burton) will deliver the wedding ring. However, Walter persists in his criticism of Goodfellow, branding him a disgraced officer unsuitable as best man. Edgar Sullivan promises his father to address the paperwork issues later, as they cross paths with Archibald Drake (Andrew Tiernan), who expresses enthusiasm about Edgar's return.
Walter's constant complaining precedes an attempted speech, allowing Drake to engage Felicia in conversation about Edgar's London days. She obligingly fetches him another drink. Grace shares with Brenda and the others her belief that her husband enjoys terrifying her with tales of her driving, prompting Brenda to note Isabel Devine's penchant for speeding too. Devine recalls driving an ambulance during the war, while Grace reveals her aspiration to become a doctor before marrying Walter.
Archibald takes Edgar aside to discuss his new role as Detective Superintendent, to which Edgar admits his father's assistance. Goodfellow's arrival prompts Sullivan to apologize, with Walter hoping for no lingering resentment. Goodfellow decides to depart, albeit he will attend the wedding. Meanwhile, Archibald voices his disapproval of Edgar's new position.
He heads to the pub for a refreshing beverage, only to have Felicia keep tabs on him. Later, she breaks the news to the circle that Archibald's cheerfulness has waned. Irene, sensing the mounting tension, decides to call it a night prematurely. Walter comes to understand that the commotion stems from his son's new employment. He attributes it all to jealousy, little knowing what lies ahead.
The following day, disaster strikes as Walter is shot once in the shoulder. Edgar arrives just in time, thwarting any further attempt on his life. At the hospital, where the air is thick with concern, Grace and Father Brown are informed of the shooting. Despite the ordeal, Walter insists that the wedding proceed undeterred by any adversity. Edgar confides in Father Brown about his father's law enforcement background and his involvement in arresting Reggie Robinson. He reveals his dad's wish to have the wedding guests screened for criminal records. Father Brown ponders over this and suggests they converse with Irene.
Sullivan, anxious for justice, seeks Father Brown's help in uncovering the shooter's identity. Stepping outside, Father Brown engages Goodfellow in a discussion about the shooting, expressing his desire to speak with the florist, Irene Frost. He also requests Goodfellow's assistance with something else.
Father Brown locates Irene at the bustling bus stop. She confesses that Walter Sullivan dismissed her last night, asserting that a convicted criminal couldn't be trusted. Irene admits she took it personally, considering how crucial this job was for her. She had believed her troubles were behind her, working alongside Felicia. Despite her plight, Irene maintains her innocence in the shooting. Father Brown suggests she seek reconciliation with Felicia and Isabel Devine, who, he notes, can be surprisingly forgiving.
Irene reveals her past—she was caught shoplifting after losing her fiancé. Felicia, moved by her story, agrees to reinstate her. Meanwhile, Father Brown and Devine consult Goodfellow, who informs them of the gun's discovery. He confirms that it was a police-issued weapon checked out by Archibald and later handed in at the station.
Father Brown seeks Archibald out in the pub, questioning him about the oversight in reporting his gun missing. Archibald admits he's being compelled into early retirement due to Edgar Sullivan's relocation to London, adding another layer of complexity to the unfolding drama.
Archibald insists that he would never have missed his mark. As Edgar catches up with Devine at the church, he relays the message that his father persists in wanting the wedding to take place the very next day. Grace echoes this sentiment, believing that the ceremony should proceed as planned. Interrupting their conversation, Archibald announces his imminent return to London.
Shortly thereafter, Eddie and Brenda arrive, presenting Father Brown with an object discovered near Sullivan's car. Upon examining it, Father Brown reveals to Edgar that the fragment is, in fact, a diamond. He speculates that the force of the golf club striking the glass must have dislodged it. Walter joins them shortly after being discharged from the hospital, stubbornly refusing to let Father Brown examine the rash on his hand.
Felicia brings flowers to Devine and presents her with something borrowed from Grace—much to Devine's surprise, it is her engagement ring. Father Brown, observing a missing diamond, seeks a private conversation with Grace about the ring and its missing gemstone. In this confidential exchange, Grace confesses to vandalizing her husband's car, finding the act strangely satisfying. She reveals her distress stemming from Archibald's revelation that Walter thwarted her aspirations to become a doctor. Walter had offered to mail her medical school application but instead discarded it in the trash, believing a wife with a career did not align with his vision. Though Grace admits she was tempted to shoot her husband, she stresses that she did not commit the act.
Edgar learns of this conversation and seeks Father Brown's assessment. The priest assures him that he is not convinced of Grace's guilt in shooting Walter. As Father Brown arrives just in time for Devine's dress fitting, Isabel Devine declares the gown hideous. She admits to having been dishonest with them, while Devine reveals that Walter will only give his blessing for the marriage if they agree to relocate to London. He threatens that Devine will jeopardize Edgar's career due to her connection to Father Brown if they remain. Devine, however, insists that she only wants what is best for Edgar.
On her final day, she yearned solely for the solace of being her true self. Subsequently, Devine engaged in a heartfelt conversation with Father Brown about Irene, whose fiancée, Alfie, had tragically passed away just two days shy of their wedding day. Father Brown, piecing together the fragments of the story, discerned the hidden truths.
As the pub buzzed with excitement, preparing to toast Edgar, Father Brown made his entrance. He intervened, halting Walter's impending toast and requesting a private audience with Irene. In a hushed tone, he referenced methanol, the silent killer of effervescence, hinting at its use in a grim act of vendetta. He deduced that she employed it two days prior to avenge Alfie's untimely demise, not by a Nazi bomb as presumed, but by Walter Sullivan's ruthless hand in the pursuit of a notorious gangster.
Walter and Edgar's arrival added a tense layer to the atmosphere. Irene recounted the fateful walk in Ealing with Alfie, the day before their wedding. In a split-second decision, Alfie shoved her out of harm's way, saving her from the oncoming police vehicle, unaware it was driven by Walter. When Walter claimed ignorance of her name, Irene's frustration boiled over, reminding him how he falsely stated Alfie was deliberately in the road.
Irene, bound by her solemn promise to Alfie, orchestrated her revenge with meticulous planning. She secured a job at Felicia's florist shop and cunningly poisoned Walter's drink. However, Walter's rejection of the poisoned wine left him with a painful rash, thwarting her initial plan. Undeterred, Irene seized the gun, biding her time until morning. Her escape was cut short by the arrival of a car, leaving her plans incomplete. Father Brown, with compassion and firmness, exhorted her to seek redemption. Yet, Irene harbored no regrets, save for the task she failed to fully accomplish. She was then escorted away.
Edgar, disillusioned by Walter's true nature, revoked his role as best man and reclaimed his wedding ring. Father Brown enlightened Devine about Walter Sullivan's deeds. Edgar, turning to Goodfellow, offered him the honored position instead. Devine joined the chapel shortly after, witnessing Isabel and Edgar's union.
Outside, Edgar confided in Archibald his intentions to lobby for his job reinstatement, vowing to excel beyond his previous achievements. Walter, displeased with his son's actions, faced a stern rebuke from Edgar, who lamented the years lost under his father's oppressive shadow. Brenda expressed her desire to accompany Eddie to London, which he warmly welcomed. Edgar further pledged to reinstate Goodfellow as a police officer. Meanwhile, Devine surprised Father Brown by announcing her decision to stay.
