Capturing Mary introduces caretaker Joe Dix (Danny Lee Wynter) to Mary Gilbert (Maggie Smith), a past luminary of the grand mansion he now tends. When Joe welcomes Mary into the dormant house, he coaxes her to share her memories of it with him.
Once inside Mary wanders the corridors of the splendid mansion where as a brilliant young writer and critic (played in her youth by Ruth Wilson) she attended glamorous soirees during the late Fifties. As she does so, she recounts to the sympathetic Joe the tale of woe that befell her within these gilded walls.
As she reflects on her heyday, remembering parties and functions with the cultural elite, she is haunted by the memory of a subtly evil man, Greville White (David Walliams), who feigned friendship, but actually brought about her destruction.
Now in later life, Mary cannot escape the fact that revisiting the past is not always a happy nostalgic journey, but rather can provide the understanding of where despair and disappointment began.