The episode kicks off with the postman, Predrag Grujicic, depositing a pile of mail on the doorstep right in front of Eliza Scarlet, portrayed by Kate Phillips. Later, Eliza and Scarlet find themselves poring over applications when Patrick Nash, played by Felix Scott, enters the scene. Clarence, embodied by Paul Bazely, explains that these applications are for their open detective position. Nash attributes the deluge of responses to his own formidable reputation. Scarlet muses that perhaps they were all drawn by whispers of her being a female detective. As they sift through the applications, no consensus emerges; they simply cannot settle on a single candidate.
Scarlet catches Ivy, played by Cathy Belton, in the act of penning a letter. Ivy claims it's to a mere friend, but Scarlet, seizing the letter, discovers it's actually addressed to Inspector William “The Duke” Wellington, portrayed by Stuart Martin. Ivy confesses she's been ignoring his last three letters. Scarlet swiftly denies being upset with him before rushing out to work, skipping breakfast in her haste.
Scarlet's thoughts drift back to The Duke’s parting words before he jetted off to New York. She prepares to immerse herself in one of his letters when Clarence bursts in, alerting her to a new client. Later, Scarlet and The Duke find themselves at the Britannia Telephone Company, discussing with Joseph Bracewell, played by Paul Clayton, ways to mitigate the damaging publicity stemming from an explosion that claimed the lives of inventor Alfred Baker and Britannia’s co-founder, George Davidson. Bracewell posits that the explosion was likely caused by a gas leak combined with Davidson's habit of lighting his pipe. He adds that the police suspect Bracewell tampered with the gas supply. Scarlet, curious, tries the telephone for the very first time.
Bracewell underscores the urgency of identifying the culpable party to salvage Britannia’s tarnished reputation. He admits to having disagreed with Davidson over the company’s expansion but categorically denies any involvement in his murder. Furthermore, Bracewell wonders aloud why he would want to harm Alfred when his telephone invention had propelled Britannia to great fortune. Outside, Nash and Scarlet find themselves at odds, each holding a vastly different perspective on Bracewell.
As Scarlet and Nash draw near, DS Phelps (portrayed by Tim Chipping) summons his carriage with an air of grandiosity, boasting of his impending ascendancy to The Duke's position. Scarlet inquiries why Phelps' superiors have yet to finalize the promotion, to which he assures her of his rightful deserving. Nash then interjects, revealing that Britannia has entrusted them with probing into the mysterious gas explosion. Phelps responds by mentioning that fire experts are still examining the crime scene, prompting Nash to inquire about conducting their own parallel investigation. Phelps, however, refuses their request, sparking Scarlet's sarcastic remark about his newfound lack of decorum despite the potential promotion. Phelps insists on being respected as the new leader, but Scarlet retorts that respect must be earned. Nash steps in to mediate, but Phelps remains unyielding.
Later, Nash broaches the topic of Scarlet's feelings towards The Duke's departure, which she swiftly denies. He urges her to cooperate with Phelps and cease her mopey demeanor, likening it unfavorably to his mother's. Scarlet, with a mischievous grin, agrees before they delve into their investigative strategy.
Betty Baker (played by Alexandra Hannant) shares with Scarlet and Nash that her marriage to Alfred lasted a mere six months before the tragic accident. Nash enquires about anyone who might have harbored ill will towards Alfred, and Betty mentions Daniel Archer, who was upset that Britannia chose Alfred's telephone design over his. She claims Alfred once told her that Daniel tried to pilfer his plans. Nash then asks about the whereabouts of Alfred's designs, but Betty professes ignorance.
Daniel Archer (Matthew Steer) introduces Nash and Scarlet to his revamped telephone plan. Nash teases Archer by meddling with the telephone components, successfully ruffling his feathers.
Subsequently, Scarlet informs Nash of Britannia's telephone's three primary flaws: cost, risk of short-circuit, and subpar sound quality. Nash notes the absence of any financial woes, grudges, or familial ties that could implicate Davidson. He proposes they shift their focus to Archer, who lacks a solid alibi. Examining crime scene photographs, Scarlet notices scorch marks opposite the bodies, casting doubt on the detectives' theory that Davidson's pipe sparked the gas explosion.
As Scarlet and Nash approached, Phelps, in an attempt to expedite his men's work, hurled insults at them. He lamented the delay caused by the fire experts, who were tied up battling a blaze at a St. Albans factory. Nash skillfully persuaded Phelps to allow them to inspect the Britannia explosion site by generously offering to credit Phelps entirely for their findings.
At the crime scene, Scarlet discovered wires lacking insulation. With a stick, she cautiously bridged two wires, sparking a flash. Her keen insight revealed that short circuits posed a threat in the Britannia phone, before dawning on her that a phone call, rather than Davidson's pipe, had ignited the gas. The caller, she deduced, was the culprit.
When addressing Britannia Foreman Mr. Underwood (Steve Furst), Nash impersonated Inspector Bates and requested the call log for September 21. Underwood, suspicious, demanded to see a warrant. Nash's confession led Underwood to summarily eject him.
Elsewhere, Scarlet delved into Alfred's Britannia contract, uncovering a starting revelation: Alfred was to receive a mere 10% of the profits, while Bracewell and Davidson each stood to gain 45%. A clause stipulated that upon Betty's arrival, Bracewell would be entitled to 100% of the profits. Later, Betty reported that her home had been invaded during her absence. Scarlet noticed that the "Inventions That Changed The World" volume II was conspicuously missing from Alfred's collection.
As Nash awaited the arrival of the Britannia night shift manager, Scarlet approached him with a theory. She speculated that Alfred might have jotted down his phone plans in the missing book. Just then, Underwood emerged from the business and boarded a carriage, prompting Scarlet to question how he could afford such luxury on a manager's salary.
Elmer Goodheart (Earl Kadic) presented them with the call log for September 21. Nash's eyes lit up as he spotted the call that ignited the gas at the workshop – it had originated from Bracewell's office. Despite this, Nash insisted on investigating Archer rather than Bracewell, who was a paying client. Scarlet argued that Bracewell stood to gain from Davidson and Alfred's deaths and had admitted to being in his office when the workshop exploded. Nash, keen on securing their fee, insisted she focus on Archer.
Later, Scarlet felt upset when Ivy tried to persuade her to speak with The Duke.
Scarlet confronts Bracewell head-on, armed with incriminating evidence. He insists he was negotiating a buyout at the United Telephone Company in Ealing when the workshop blew up, querying how he could profit from a telephone that ceased to exist. She counters by revealing Alfred had duplicated his blueprints. Demanding immediate possession of Alfred's plans or the termination of Nash's services, Bracewell leaves no room for negotiation.
Upon her return to the office, Nash hands Scarlet a telegram from Bracewell, tersely dismissing their employment. Scarlet informs him that Bracewell's alibi has been verified but before departing to persuade Bracewell otherwise, Nash sternly instructs her to adopt the demeanor of a seasoned businesswoman in his absence. Meanwhile, Clarence breaks the news to Scarlet about Archer withdrawing substantial sums from his account.
Scarlet keeps watch as Archer engages in hush-hush negotiations with Underwood. Sneaking into Archer's office after him, she confronts him with Alfred's stolen tome containing the telephone plans. She accuses him of attempting to pass off Alfred's invention as his own. Archer retaliates, asserting that Alfred was his apprentice and he deserves a slice of the recognition. Scarlet threatens to involve the authorities unless he hands over whatever Underwood gave him. He dumps a stack of papers on the desk. Scanning through them, she discovers they are transcripts of phone conversations between Alfred and Anderson.
Later, Scarlet confirms to Nash that his suspicions about Archer were spot on. They pay Underwood a visit, who denied any connection to the phone call that led to the explosion. Scanning the September 21 logbook, Scarlet notices his handwriting and confronts him with a blackmail letter, threatening exposure unless he documented a call originating from Bracewell's office. Underwood claims the call was made from a public phone in Farringdon.
Nash unravels the identity of the caller, while Scarlet compares the handwriting from the blackmail letter to the call log, confirming they both belong to B.L. Underwood reveals that Elizabeth Landing, alias Betty, left six months ago to marry her fiancé, who worked for Britannia, explaining the sign-off. Scarlet ponders why Elizabeth chose the initials B.L., which Nash clarifies, pointing out her true identity as Betty.
Betty openly confesses her desire for Bracewell and Davidson's demise, while exonerating Alfred from her vengeful intentions. She meticulously orchestrated the workshop to trigger an explosion upon receiving a phone call. With tears streaming down her face, she insists that Alfred's actions merited more than just a mere 10% of her wrath. Meanwhile, Nash coerced Scarlet into handing over the evidence to Phelps, hoping to secure the accolades for solving the case.
Afterwards, Nash abandons Scarlet to conduct interviews for the detective position, cunningly installing a phone in their shared office. Scarlet accuses him of trying to spy on her, to which he categorically denies. She counters, pointing out the phone's inability to make international calls, thus rendering his denial suspect.
Elsewhere, Scarlet and Ivy mend their fences, with Scarlet confessing her distress over The Duke's departure. The scene shifts to a nostalgic flashback where a youthful Eliza (Laura Marcus) encounters a youthful William (Matt Olsen) for the very first time, kindling a burgeoning connection. As the episode draws to a close, Eliza pens a heartfelt letter to William, the ink on the paper reflecting the burgeoning emotions of their nascent bond.
