Roy Courtnay, portrayed by Ian McKellen, is a cunning and aging con artist from Britain. Together with his partner in crime, Jim Carter, who plays Vincent, they expertly manipulate unsuspecting victims to gain access to their wealth through elaborate schemes and false identities. In the year 2009, Roy targets Betty McLeish, a former Oxford history professor played by Helen Mirren, who is still mourning her late husband and has accumulated savings exceeding £2 million.
Feigning a severe knee injury, Roy manages to charm his way into Betty's home, gradually persuading her to open a joint offshore investment account under false pretenses, all while plotting to siphon off her funds. In tandem with this scam, Roy and Vincent run a dubious investment scheme involving Bryn, a character played by Mark Lewis Jones, who is duped into believing he can quickly double his investment after observing Roy feigning a significant contribution alongside a fake Russian investor.
As their tangled webs of deceit unfold, one of their supposed Russian allies—a Polish butcher—demands more compensation, prompting Roy to resort to brutal means to silence him. Accusing Roy of betrayal, Bryn trails him during his rendezvous with Betty. In a dark turn, Roy leads Bryn to Charing Cross tube station, where he coldly dispatches him by pushing him into the path of an oncoming train, effectively eliminating a threat to his scheme.
Their paths lead them to a vacation in Berlin, where Roy pretends to be oblivious to German language, although it's evident he understands it. During one encounter, Betty’s grandson, Russell Tovey, reveals the history of a man named Roy Courtnay—a soldier who fell victim to a Nazi operative in 1948. This piques Betty's curiosity until Roy reluctantly admits that he had previously gone by the name Hans Taub, the German translator, who was involved in a much darker past, stealing Roy's identity to create a new life.
As they contemplate transferring their investment, Vincent cautions Roy to reserve some funds for Betty’s needs; however, Roy, ever the fraud, intends to leave her high and dry, consistent with his prior scams. Trouble brews when Roy discovers he has misplaced the necessary means to access their joint account and returns to Betty's home to retrieve it, only to find that she has stripped the place bare.
In a shocking confrontation, Betty reveals her true identity—Lili—and recounts their terrifying past as German teenagers ensnared in a harrowing tale of betrayal, violence, and survival during the war. Lili recounts the devastating toll Roy’s alter ego exacted on her family, including her father's execution and her own suffering from Hans's brutal actions. She also uncovers the lengths her grandson, Michael’s, partner Steven has gone to track Roy’s criminal past through DNA evidence, exposing the full extent of Roy's deceit.
With her plans finally in motion, Lili drains the joint account, leaving Roy with barely enough to repay his previous victims. Just as she is about to depart, Roy, in a desperate bid to regain control, locks her in and attacks, yet she possesses the strength to thwart him. As she escapes, Roy is confronted by Bryn’s accomplices, culminating in a vicious assault on him for his betrayals.
In an ironic twist of fate, weeks later, we see Roy left in a hospital bed, mind barely intact, while Lili thrives surrounded by family, illustrating the ultimate outcome for both their lives—a tale marked by betrayal, deception, and resilience amidst the ruins of their dark history.
