As the third episode of "The Law According to Lidia Poet" unfolds, the Poet family gathers for a portrait, marking a day filled with anticipation. Jacopo teases Lidia about Andrea, hinting at a whiff of infidelity lurking in their relationship, while Marianna's gaze drifts aimlessly, her eyes constantly drawn to the window. Yet, it is not Lorenzo who captures her attention but Vittorio, Enrico's friend, who drops a bombshell—he claims to have taken his father Antonio's life.
Lidia, poised to wed Alberto, Vittorio's brother, eagerly wishes to accompany Enrico to the scene. However, Enrico cites the judicial writ that bars her from entering the courtroom. Matilde, Vittorio and Alberto's mother, is incredulous. She insists that Alberto is in Nice on a business trip, accompanied by his pregnant wife. Vittorio admits to a heated altercation with Antonio the previous night, saying he smoked opium to soothe his frayed nerves. Upon waking, he found a razor in his hand and Antonio lifeless.
Lidia and Jacopo visit the murder scene, a house once filled with her childhood memories, now owned by the Muraros. Intricate trail marks on the wooden floor catch her eye, leading from one room to another. Lidia deduces that someone—perhaps Vittorio—was dragged forcibly from one room to another. Jacopo discovers a lengthy strand of hair, unmistakably not Vittorio's. Their investigation is abruptly halted when the prosecutor catches them trespassing.
Enrico listens intently to his sister's arguments for Vittorio's innocence. The killer, wielding the razor, must have stood perilously close. If so, Vittorio would be splattered with blood—a detail notably absent. Despite this, Vittorio has confessed. Meanwhile, Lidia receives a thoughtful gift from Andrea, currently exploring the wonders of Constantinople. The next morning, she exchanges it for a sleek new bicycle.
Alberto confides in Lidia, revealing that Vittorio's conflict with his father stemmed from his opium addiction. He also mentions Vittorio's girlfriend, Beatrice, a humble washerwoman whom Vittorio occasionally invited to the house for their opium-induced escapades. Intriguingly, one of Matilde's precious necklaces is missing. Lidia seeks out Beatrice, who maintains she was with Vittorio that fateful night, witnessing everything with her own eyes.
Teresa gasps in disbelief upon seeing Lidia's new bicycle. Elsewhere, Lidia catches Lorenzo and Marianna in a tender moment, cautioning them to be more discreet. Lidia and Jacopo delve deeper into the mystery, Jacopo suggesting that Beatrice might have stolen the necklace, encountered Antonio, killed him in a panic, and framed Vittorio by dragging his body to the scene. However, this theory stumbles upon a crucial hurdle: Vittorio is not a lightweight, and Beatrice, despite her courage, lacks the physical strength to maneuver his body.
Lidia and Jacopo arrive at the crime scene armed with the former's vintage key. Upon entering, they stumble upon a gory discovery—bloodstains mar a book perched on a distant shelf, an inexplicable sight given the body's remote location. Intrigued, they tug on the book, revealing a hidden door that swings open with a creak. Lidia's mind flashes back to childhood memories of eerie sounds emanating from that very wall, memories that were once dismissed with derisive laughter.
Inside the concealed chamber, the air is thick with the scent of fresh tobacco, and a cryptic message awaits on a scrap of paper. Impelled by urgency, Lidia desires to inform Enrico of their findings. Marianna, however, intercepts her with news that Vittorio has hanged himself in prison—news that's soon proven false when it's revealed he's still breathing.
Lidia rushes to Enrico, unveiling the truth in its entirety. She unfolds a train ticket from Collegno, hinting at a clandestine journey, and confides in him about the secret room—a secret that the murderer is privy to. Suspicions converge, pointing fingers at Alberto. His alibi of a trip to Nice crumbles, yet the motive for murdering Antonio remains a mystery.
Alberto is apprehended, while Vittorio is freed from his unjust confines. The revelation unfolds: Alberto was denied a crucial business loan by Antonio, who stood in the way of his entrepreneurial dreams—a dream he had once shared with Lidia at the episode's dawn. To further his nefarious plot, Alberto bribed Beatrice to deceive and steal a necklace.
Lidia seeks clarification from Enrico about their father's decision to sell the family home. Enrico confesses that their father's miscalculated investment plunged them into a abyss of debt. Within the secret room, Lidia discovers a letter from her father, urging her to marry Alberto, a union that would cancel their debt in Antonio's eyes.
Enrico is consumed by shame regarding his father's actions. Burdened by guilt, he resolves to challenge the royal decree that bars Lidia from the court, vowing to restore her rightful place. Their conversation concludes in a heartfelt embrace.
Later, Lidia encounters Andrea and cordially invites him to dinner. On her way back, she catches Jacopo slipping a note to a small child. Her curiosity piqued, she tails Jacopo to a clandestine meeting with Nicole, the scene dripping with intrigue and subterfuge.
