How well can you really know a person? Maybe we never know anybody’s true self. We can chat to them, love them, sleep with them, but who can fully understand the stuff stored in their brains that is terrified to come out? In Mrs Wilson, based on the true story of actor Ruth Wilson’s grandmother, these seemingly academic questions are taken to their extreme with devastating results.

Alison Wilson (Ruth Wilson) is a wife and mother, living in a quaint London suburb in 1963. Her husband Alec (Iain Glen) is a writer and ex-secret service agent, polishing off his latest novel. Then, out of nowhere, Alec collapses and dies. And before the body even has a chance to go cold, a woman comes to the door claiming to be Alec’s wife.

Iain Glen plays the mysterious agent Alexander Wilson

On first glance of Alison, she’s a picture of ‘50s normality – like she’s just walked in from a Douglas Sirk film. She organises everything for her husband and when that’s thrown out of balance, her world starts to crumble. The whole episode is spent trying to regain that balance again, even deluding herself that this ‘wife’ can’t possibly be his wife. But as Alison thinks back to her life with Alec, meeting him at the Secret Intelligence Service (MI6) during the war and enduring months of him being away, she begins to question her entire marriage.

Wilson delivers an excellent performance as her own grandmother, trying to hold a British stiff upper lip in the wake of everything that’s happening to her. She’s stern, takes no nonsense – even slapping her son when making a navy joke about women (‘You treat women with respect!’). But that hard shell starts to melt, and she feels compelled to investigate further into who husband really was.

Though the script is sometimes burdened with the occasional aphorism and predictable cliché (‘one needs to have faith’), writer Anna Symon creates a very personal spy drama – in direct contrast to recent BBC thrillers like Killing Eve and Bodyguard – not only investigating her husband, but herself too. How could she not have known? And will this investigation of her supposed husband give her peace, or just more grief? In any case, Symon has hooked us for the next three weeks.

Mrs Wilson will air on BBC One at 9pm on Tuesday 27th November