As the curtains rise on this installment of Father Brown, Cardinal Papillon (portrayed by Colin McFarlane) makes a clandestine visit to Father Brown (played by Mark Williams). With a grave tone, Papillon imparts confidential information, emphasizing that this matter must remain under wraps and away from police sight. He then unveils the theft of the Cup of Calabria, a priceless relic held in the highest esteem by the Vatican. He suggests enlisting the aid of Hercule Flambeau (embodied by John Light), whom Father Brown considers his spiritual guide. Papillon urges Flambeau's involvement in retrieving the stolen artifact.
Isabel Devine (portrayed by Claudie Blakley) introduces the subject of bell ringing to Father Brown, just as a disheveled and intoxicated Flambeau stumbles in. In a hurry, Father Brown conceals Flambeau from Isabel's sight, granting her the remainder of the day off as a pretext. Once she exits, Flambeau inquiries about the reason for his unexpected presence. Father Brown unfolds the tale of Father Vincent Lazarus, recently excommunicated, who stole the Cup and fatally wounded a guard during his escape.
Flambeau expresses astonishment at the Vatican's reliance on him. He also notes the irony of Father Brown hearing his confession one day and then urging him to violate the seventh commandment the next. Father Brown reassures him that guiding Flambeau towards redemption remains paramount, acknowledging it as a personal plea from His Holiness. However, Flambeau persists, vowing that Flambeau's soul remains his primary concern, believing it to be in distress. Flambeau doubts his efficacy in the case, yet Father Brown insists he needs Flambeau, unsure of his own capabilities without him. They aim to recover the cup within a week, as it is slated for use in a mass honoring the Grand Duchess of Luxembourg. Flambeau, albeit reluctantly, agrees to assist Father Brown one final time.
Brenda Palmer (played by Ruby-May Martinwood) catches them in an intimate conversation, prompting Father Brown to discreetly silence her. Flambeau cuts in, stating the situation is beyond his control. In private, Flambeau reveals the existence of a hush-hush auction organized two days prior at The Cedarmont Hotel, scheduled for that very afternoon.
Dame Lydia Adams (embodied by Christina Cole) possesses a letter pertaining to Calabria. When Brown and Flambeau encounter her, Father Brown acknowledges a vague familiarity. Isabel returns, having forgotten her purse, and discovers incriminating evidence on the floor, just as Brenda enters the scene. Ronnie Larkin (played by Danny Szam) demands to see their invitations. Flambeau claims to have left his at home, while Father Brown claims to be there for the purpose of authenticating the item. Dame Adams seeks his expertise in verifying its authenticity. Father Brown, granted permission, confirms its genuineness. He inquires about her intense desire to possess it, prompting Charles Radler (played by James Sheldon) to wonder if the cup holds healing powers. One attendee attests to its undeniable prowess. Brenda attempts to cover for Flambeau and Father Brown, inadvertently revealing Flambeau's presence. Isabel grows distressed, fearing that Edgar might find out and call off their wedding. Brenda cautions her to keep silent. The bidding for the cup commences amidst heightened tensions.
The gentleman, seemingly ailing, emerges victorious in the cup's contest, yet his triumph is fleeting; he collapses and passes away shortly thereafter. Dame Adams ponders aloud, suggesting it as an act of divine providence. Meanwhile, Isabel engages in conversation with Goodfellow (played by John Burton) and Edgar Sullivan (portrayed by Tom Chambers) within the confines of the jail. Edgar is swamped beneath a pile of paperwork, prompting Isabel to consent to their excursion the following day. Her gaze falls upon a wanted poster depicting Flambeau adorning the wall, causing her to exclaim his presence aloud. Father Brown urges the group to alert the authorities promptly. Ronnie decides to consult the client regarding the cup's disposal, as Flambeau discreetly unlocks the window, preparing his exit.
The client concurs to sell the cup to Dame Adams, albeit at the final auction price. Ronnie urges her to decide swiftly, warning that his departure looms amidst the impending police arrival. He provides her with the seller's account details, while Dame Adams contacts Giles with instructions. Upon entering the room, Flambeau lifts the cloth only to discover the cup's absence. He accuses Ronnie of orchestrating the deception by covering it up. Father Brown notices the absence of the other bidder. Ronnie manages an escape, prompting Father Brown to deduce that one of those men could very well be Lazarus.
Flambeau speeds off in a car, chasing after Ronnie with Father Brown in tow. However, their pursuit is foiled by an obtrusive tractor. Elsewhere, Brenda undergoes questioning by Edgar. Ronnie is shot by an unknown assailant and ejected from the moving vehicle. Father Brown rushes to his aid, with Ronnie blaming Lazarus for his plight, claiming the latter shot him for granting entry into the auction. Ronnie mentions his visit to Helmey Wood before succumbing to his injuries. Father Brown begins to berate himself for Ronnie's fate, but Flambeau contends that they did not pull the trigger. While Brown offers prayers, Flambeau reflects on his late father.
Together, Brown and Flambeau venture into the woods in search of Lazarus. Flambeau steadfastly denies being suicidal or insane. Father Brown mentions Flambeau's earlier words about being a burden and acknowledges his ongoing grief over his father. Flambeau insists he is well. Their search leads them to a secluded house in the woods, where Father Brown discovers documents detailing information about Dame Lydia Adams.He asserts that she is a substantial benefactor to the Vatican, prompting their visit to inform her that she might have fallen prey to a deceptive scheme. Father Brown and Flambeau are convinced that Lydia, knowing full well, would take the bait and place a bid of 70,000 pounds. With a heavy heart, she reveals that her husband and daughter perished in a tragic car accident last year, leaving her with not a single day devoid of suicidal thoughts since then. Despite these dark musings, Lydia confesses she lacks the courage to carry them out. She had harbored hope that partaking of the cup might finally alleviate her suffering.
Lydia unburdens herself to Father Lazarus, expressing doubts about her worthiness to receive the cup's supposed blessings. Father Brown, sensing an inconsistency, inquiries how she could possibly know her unworthiness unless she had already partaken of it. To everyone's astonishment, Lydia produces the cup, claiming she found it on her doorstep upon returning from the auction. Father Brown surmises that it was always his intention for Lydia to possess it, speculating that Lazarus only stole it because their timely arrival interrupted his plans. He further posits that Lazarus deliberately set Lydia up to be caught, thereby ensuring the Vatican would never accept another donation from her again.
Before taking his leave, Father Brown reassures Lydia that his door at St. Mary's is always open to her. As Brown and Flambeau make a hasty retreat with the cup, Father Lazarus confronts them at gunpoint outside. In a swift move, Flambeau disables him and informs Brown that their mission has concluded. He disables Edgar's tire with a shot before speeding away. Father Brown warns them that Lazarus is a murderer, yet remains silent about the artifact's significance. Flambeau, agitated, narrowly avoids a collision with a tree. Brown promptly calls Papillon to inform him of the cup's whereabouts and Lazarus's arrest at the Kembleford police station. Intriguingly, Brown notes that Lazarus quoted from the King James Bible rather than their Douay-Rheims version. Papillon cuts him short, instructing Brown to safeguard the cup until his arrival.
Meanwhile, Father Brown discovers Isabel in tears, soon joined by Brenda. He recounts the legend of the Cup of Calabria, crafted from the true cross, believed by some to possess miraculous powers. He reassures them that it will be returned to its rightful place at the Vatican.
He earnestly desires their companionship at St. Mary's, as Papillon arrives at the station, intent on conversing with Goodfellow regarding Lazarus. Upon entering, he claims ignorance of the inmate in the cell. Father Brown utters a prayer, his voice resonating over the sacred chalice. Suddenly, the genuine Lazarus steps forward, brandishing Andronicus's knife and compelling them to take a seat. Father Brown comprehends that Lazarus, posing as deceased, had seized the chalice under cover of darkness.
Lazarus confesses his admiration for Father Brown, asserting that they are both rebels at heart. He boasts of leveraging his intellect to wage a moral crusade, declaring the church corrupt and in dire need of an internal purge. Devoting his life to this mission, Lazarus admits his presence isn't to reclaim the chalice but to accuse Father Brown of thwarting his plans, a transgression that will not be overlooked.
Brenda and Isabel intervene, assaulting Lazarus. As they scramble to escape, they abandon Father Brown in Lazarus's company. Meanwhile, Edgar discusses the investigation with Goodfellow, who informs him of Papillon's visit. In a rush, they exit the station.
Lazarus attempts to incinerate the chalice, but Flambeau returns just in time to extinguish the flames. Edgar and Goodfellow arrive, catching Flambeau and Lazarus in the act. Papillon protests, asserting that the Chief Inspector is apprehending the wrong individual. In a desperate lunge, Lazarus aims to stab Father Brown but instead pierces Flambeau's abdomen. Brenda quickly dilutes the contents of the chalice. Father Brown, acknowledging Papillon's assistance, offers him a sip from the blessed vessel.
Flambeau reveals his return was to confess that Father Brown's insights about him were correct. As Father Brown administers the last rites, he later visits Lazarus in jail. Lazarus reveals that the trick he employed was learned from a yogi in India long ago. Father Brown seeks insight into Lazarus's seminary years and his infiltration of the Vatican. Brown presses for the reason behind his intense hatred for the church. Lazarus replies cryptically, hinting at a tale for another occasion. He expresses regret over the unintended victim, Flambeau, acknowledging that Father Brown was the intended target. However, Lazarus is relieved that he spared Father Brown's life, finding a worthy adversary rare.
He declares the game already afoot. Subsequently, Father Brown is granted a brief visit with Flambeau in the hospital. They ponder whether the chalice might have been instrumental in saving Flambeau's life. Papillon arrives with extradition papers, instructing Goodfellow to uncuff Flambeau. He informs Flambeau of his temporary custody but assures him of eventual freedom. The Vatican has a proposition: Lazarus stole numerous artifacts from the church. The newly founded Reclamations Department aims to retrieve these items, and they seek Flambeau's assistance. Flambeau eagerly inquiries about the start date. By the time Edgar arrives, Flambeau has already departed.
