As the curtain rises on the climactic episode of "Miss Scarlet," Chief Inspector Blake (Tom Durant-Pritchard) engages in a somber discussion with DS Phelps (Tim Chipping) and their colleagues, focusing on a deceased individual now resting at the morgue. The responsibility for leading this investigation falls to Irwin (Filip Radovanvic), to whom DS Phelps and Jones eagerly propose scrutinizing a roster of known acquaintances. Given the deceased's checkered past, they anticipate encountering some recognizable names. In a confidential exchange, Phelps quizzes Blake on his rationale for placing Irwin in charge, hinting at his own frustrations at being sidelined. Blake rationalizes that the victim hailed from Holloway, an area where Irwin boasts a network of informants. Phelps vents his grievances, noting he hasn't spearheaded a case in months, accusing Blake of personal motives. Blake refutes these allegations, asserting that Phelps is manufacturing conflicts where none exist. Phelps warns of potential rebellion among the ranks, to which Blake replies with a nonchalant acceptance of the risk.
Later that evening, Phelps convenes with fellow officers, rallying them to demand more respect from Blake. His proposal, however, meets with indifferent responses. The following morning, Eliza (Kate Phillips) expresses consternation to Ivy (Cathy Belton) over the disappearance of her bag and notebook. Ivy, preoccupied with wedding preparations and Mr. Potts' impending move-in, reveals Potts has misplaced the bag in Eliza's bedroom, much to Potts' own exasperation upon discovering it.
Eliza subsequently visits the police station, where she presents Blake with another invoice and inquires about any pending cases. Blake assures her there are none, but offers her the latest issue of Hayward's Forensic Science as a gesture of gratitude for showing Sophia around her office. A chance encounter with Phelps in the hallway leaves her sensing his discontent. Seeking solace, she confides in Detective Fitzroy (Evan McCabe) regarding Phelps' demeanor.
Their conversation is abruptly halted by Commissioner Fitzroy (Stephen Boxer), who informs his son of an impending dinner with his mother. Fitzroy introduces Eliza to his father, Sir George Fitzroy. Once George departs, Detective Oliver Fitzroy confides in Eliza that the dinner is an attempt to mend fences with his estranged father. Marianne (Robin Weaver) joins them for the evening meal, where she expresses concern for Oliver's well-being. George's abrupt departure prompts Marianne to inquire about "Miss Scarlet," sparking a tense exchange until Oliver persuades George, on Marianne's birthday, to lighten up. George eventually excuses himself, prompting Marianne to request Oliver escort her home.
Ivy tasks Eliza with creating samples for the morning, emphasizing the significance of the 30 guests they anticipate. At the office, Eliza shares insights gleaned from the forensic science book gifted by Blake with Clarence (Paul Bazely). Marianne visits Eliza to discuss a delicate matter, admitting that she had considered visiting before, though she admits this with hesitation in private.
She elucidates that she cannot turn a blind eye to her husband's infidelity. Just last week, George received an invoice from a florist nestled in Greek Street, revealing weeks of consistent purchases of red roses. Marianne is desperate for the truth. Eliza, however, expresses her reservations, stating that taking the case would place her in a precarious position. Despite this, Marianne implores Eliza to take it on.
Later that evening, Eliza witnesses George Fitzroy presenting flowers to a man. The following morning, she relates this incident to Clarence. Shortly after, the door creaked open, and Eliza observed Fitzroy departing. Moments later, the young man also exited the premises. Eliza tails him to the Weeping Willow, where she discovers him abandoning a bag. In present times, Eliza confides in Clarence, believing that the man left the bag for another individual. Clarence suggests she drop the matter and inform Marianne that she saw nothing incriminating about her husband.
Meanwhile, Oliver grabs a newspaper, feeling a lingering sense of being tailed. Confronting the suspected follower, he finds himself assaulted by him and another accomplice, who proceed to rob him. Blake later visits Oliver at the hospital, where Oliver reveals that the stolen pocket watch was a cherished gift from his mother for his 21st birthday. Marianne arrives shortly thereafter, and Blake assures her he will uncover the perpetrators. He then informs Phelps and the others of Oliver's condition: a concussion and three broken ribs. Blake demands a list of individuals Oliver has arrested and charged in the past two years, warning Phelps of dismissal if resentment surfaces.
Eliza arrives and learns of the attack, feigning ignorance alongside Marianne. Blake hopes Eliza will come forward if she knows anything about the incident. After discussing it with Clarence, Eliza explains her reasons for withholding information, only to be confronted by armed men who force them back into the office.
One of the men inquiries if Eliza has listened to the contents of a recording, to which she steadfastly denies any knowledge. They accuse her of following the Police Commissioner and witnessing him with Aleksey (Joakim Tasic). They demand the bag, and though Eliza initially denies having it, she eventually concedes possession but insists she hasn't listened to its contents. She promises to retrieve the bag and leave it in the unlocked office the next night. They grant her until then.
Alone, Eliza admits to Clarence that she has no clue where the bag is. Elsewhere, George Fitzroy discusses the attack with Inspector Blake, pondering if Oliver was merely in the wrong place at the wrong time. He doubts his son has made enemies, prompting Blake to inquire about George's own adversaries. George doubts he has any and accuses Blake of fruitless pursuit.
Eliza and Clarence devise a plan for the morning. Upon entering her home, Eliza finds Miss Scott (Lindsay Bennett-Thompson) and a man awaiting her, the tension in the air thickening with each passing moment.
Scott announces that Ivy has just stepped out for the night, her departure timed perfectly. Eliza recalls bumping into her en route to fetching her bag, suspecting she was a mere distraction, diverting her from securing the prize. Miss Scott, with urgency in her voice, insists that Eliza ceases her inquiry into Commissioner Fitzroy forthwith. Phelps informs Blake of a wall of silence; not a whisper from anyone, and he's yet to confer with his uncle.
Blake and Phelps pay a visit to Phelps' uncle, Dylan Cooper (played by Steve Hartley). Phelps mentions a whisper he caught—a bounty on Cooper's head. Blake, persuasive as ever, argues that aiding them would be a debt Cooper owes. Cooper, ever the font of street wisdom, divulges info about a newspaper vendor at Old Pitts Lane, a man privy to the comings and goings of every constable in town. Oliver once bought a paper from him and, according to Cooper, was being tailed.
As they depart, Phelps confides in Blake his plan to track down the vendor. Blake commends Phelps for sharing such a valuable lead with a senior officer, displaying commendable character. He entrusts Phelps with the task of finding the crucial witness. Meanwhile, Eliza urges Blake to accompany her. Phelps locates the newspaper vendor and witnesses another man hassle him before they both depart with Miss Scott.
Eliza leads Blake to the site where she witnessed George and Aleksey. Inside, they rummage through, eventually uncovering a wax cylinder. Playing it on Blake's phonograph, they hear a Russian voice mentioning Aleksey's meeting with a gray-haired compatriot, hinting at a plot to eliminate the Commissioner. Phelps bursts in with news of the woman and the red-haired thug, whom he tailed to some derelict warehouses on Blackbird Lane, aligning perfectly with the recording's revelation.
George Fitzroy arrives at the warehouse to meet Aleksey but finds himself at gunpoint. Blake and Phelps arrive just in time to thwart the execution. Gunfire erupts from above, sparking a fierce shootout. Scott enters, commanding the thug to lay down his arms. Eliza arrives moments later. Scott unveils her true identity: an agent of British Intelligence for Her Majesty's Government, codenamed the Governess, operating under QD3, the Queen's Department's most covert tier.
QD3 had been aware of the Russians' machinations to compromise the Police Commissioner. Aleksey's mission was to uncover security protocols for the new Royal Naval ship under construction, a vessel of immense technical interest to the Russians. The Russians discovered that Eliza had blown their cover. Scott returns Oliver's possessions, explaining that her men had to eliminate evidence to protect their operation. Phelps gripes about Oliver Fitzroy's brutal beating, but Scott assures that the Commissioner will retire peacefully to the countryside. Blake praises Phelps for his stellar work.
Eliza and Blake engage in a heartfelt conversation about their mutual acquaintance, Miss Scott, while the joyful news unfolds that Mr. Potts, portrayed by Simon Ludders, ties the knot with Ivy. Oliver, with a renewed sense of well-being, shares with Eliza his improved condition and the impending retirement of his father. As their chat deepens, Clarence joins their circle, clutching his third drink with a cheerful demeanor. The gathering gains another layer with the arrival of Detective Phelps, who regales them with tales of commuting for free. Fitzroy chimes in, explaining that Blake's daughter, unfortunately unwell, could not attend the festivities. Mrs. Potts, sensing the impending conclusion of the gathering, gently reminds Ivy that it's time to depart. Ivy, ever thoughtful, promises Eliza a postcard before bidding her farewell, gradually leaving Eliza in a tranquil solitude.
Amidst this serene ambiance, Eliza's gaze falls upon the book thoughtfully bestowed upon her by Blake. Intrigued, she decides to pay him a visit, armed with a wedding cake as a gesture of goodwill. She places the cake at his doorstep, but Blake, sensing her presence, warmly invites her inside. With a mixture of curiosity and anticipation, Eliza accepts his invitation, stepping into his home.
