Episode 13 of Watson kicks off with a cliffhanger, as the Croft Twins' lives teeter on the edge of existence. The group is faced with an agonizing decision: which twin to save.Lubbock stands in disbelief, uncertain of their capacity to make such a monumental choice. Watson, however, possesses a coin that once belonged to Sherlock and decides to entrust the twins' fate to the flip of a coin, an act steeped in both tradition and desperation.
Adam emerges victorious from beneath the surgeon's knife, yet Stephens' condition remains perilous. Watson clings to hope, knowing they have a window of two to three days to devise a cure for Stephens' plight. Time is a precious commodity, its passage counted in heartbeats.
Shinwell returns to the hospital, a glint of cunning in his eye, convinced there's a strategic maneuver to be made. Moriarty's agent emerges as the weak link, her daughter Ashleigh's fragile health a leverage point. He approaches her, offering a lifeline to escape Moriarty's clutches.
The plan unfolds: in exchange for treating Ashleigh's condition, "The Rep" must divulge the location of Moriarty's lab, where the adeno-associated virus—the elixir for Stephens' ailment—is rumored to be held. The Rep, seeing an opportunity for redemption, agrees to the terms. Watson and his team swing into action, their efforts focused on finding a cure for Ashleigh, whose battle with brain tumors since the age of nine has taken an aggressive turn. Chemotherapy is the prescribed course, but Ashleigh must gain weight to tolerate the treatment. Alas, weight gain eludes her, and Watson refuses to foster false hope, rejecting Derian's suggestion outright.
Amidst this tense atmosphere, Ashleigh's oncologist, Laila Bynum, arrives, her dedication to her craft palpable. She underscores the urgency of the moment, urging Watson to act swiftly. Just as tensions peak, Ashleigh suffers a seizure, her body convulsing in distress. Concurrently, Stephens' condition takes a turn for the worse. These seizures, however, bear no resemblance to her cancer's symptoms, marking an ominous new chapter in Ashleigh's fight for survival.
Amidst the unfolding drama, Dr. Derian finds herself confronted by Moriarty, who presents his business card, sealing their collaboration. Overjoyed to partner with him, she urges Moriarty to expedite his efforts and expedite the removal of essential items from the lab, mindful of Watson and his team closing in on their location.
Meanwhile, Shinwell's condition deteriorates rapidly, with vicious burns creeping across his back. Despite this, he persists in his collaboration with Watson, acknowledging the immense personal struggles Watson is enduring. As events unfold, The Rep and Shinwell stake out the lab, plotting to infiltrate and secure the cure. Just as The Rep prepares to enter, a reveal unfolds: her true identity is Hannah.
Derian's tip-off alerts Moriarty's men, who arrive with armed guards, ready to thwart Shinwell and Hannah's planned break-in. With little option left, they reluctantly abandon their mission.
Back at the lab, Watson diagnoses Ashleigh with a genetic condition known as MCAD. The meal-replacement shakes they'd been administering contained ingredients like coconut oil, which Ashleigh's body is unable to process. This revelation explains her nocturnal seizures, necessitating a switch to corn starch. Once she gains weight, chemotherapy can commence, potentially leading to her cure.
Watson informs the team that Ingrid Derian has been functioning as a double agent throughout, divulging everything to Watson. She admits to sabotaging the vector, causing the entire operation to unravel. Working alongside Watson, she's engineered Moriarty to fall ill in the same manner he did to the Croft twins.
Moriarty soon appears at the clinic, where Watson outlines the severe symptoms he's poised to endure, starting with blindness and escalating further. Watson proposes a deal: the virus to cure Stephens in exchange for Moriarty's life. Moriarty, faced with this ultimatum, eventually concedes.
How Watson's journey concludes unfolds with a tapestry of resolutions and revelations. Stephens finds healing, as Lubbock enters the scene, confessing her affection, while Ashleigh experiences a gradual improvement in her condition. Shinwell too emerges cured, and Derian arrives, seeking forgiveness from Stephens. Yet, Stephens declines to collaborate with her again, a decision easily understandable, prompting her departure from the clinic.
Moriarty's blindness is reversed, but Watson confronts his adversary, demanding clarity on his actions throughout the season. Receiving no satisfactory explanation, Watson acknowledges that Moriarty's cure is incomplete. Though his sight has been restored, Watson realizes Moriarty is still entangled in the throes of his darker days. Without the closure he seeks, Watson resolves to let Moriarty perish in his bed, a grim fate he prepares to accept.
Watson carries out his decision, allowing Moriarty to slip into the realm of eternity, leaving Mary to grapple with the aftermath. The narrative then skips ahead several weeks, revealing a new chapter in Watson's life—he is now entangled in a romantic relationship with Laila, Ashleigh's compassionate oncologist.
