Although growing old is the most common of experiences, there are surprisingly few good films about old age. Maybe because there's no audience. The young are too busy being young to be interested in those with gray hair. And the people over 50 who I know shudder at the thought of watching comedies about cute bucket-listers or dramas where the aged spend their days grappling with disease, death and loss.

My own heart sank when I first heard about Elizabeth Is Missing, a new television film on PBS' Masterpiece channel that stars Glenda Jackson as a woman battling dementia. Even when a hyper-critical London friend told me I should watch, I remember thinking, "Sure, if I live to be a million." But somehow I wound up with time on my hands, and guess what? Elizabeth Is Missing was not what I feared. Based on a novel by Emma Healey, this BBC production is a strangely cockeyed psychological thriller — a Disease of the Week movie crossed with Memento.

Jackson plays Maud, a proud woman whose grasp of reality is slipping away — post-it notes line her walls and pockets to remind her of everything. If she feels bound to anyone, it's her friend Elizabeth, with whom she likes to garden.

One day, Elizabeth doesn't turn up for their meeting at the local thrift shop, and Maud is beside herself. Elizabeth has vanished — yet no one else seems fussed by this. Not Maud's beleaguered daughter, Helen, who looks after her mum and gets little kindness in return. Not the local police. Not even Elizabeth's son, whose dark nature scared his mother.

As Maud tries to trace Elizabeth's movements, she plunges down the rabbit hole of memory. She keeps recalling — no, make that reliving — the great trauma of her teenage years: the unexplained disappearance of her stylish older sister, Sukey.

As these time periods merge in her head, Maud must solve two mysteries: What happened to Sukey decades ago and what has happened to Elizabeth now? Meanwhile, she keeps forgetting the basic facts of everyday life — and then pretending she hasn't.

When you're young you can play Hamlet or Hedda Gabler or Blanche DuBois, but aside from King Lear — whom Jackson actually played on stage in 2019 — most roles for the old aren't very juicy or challenging. But playing a character with dementia is. You get to inhabit blurring layers of consciousness, get to slide from warmth to abusiveness to humor at warp speed, and get to register how it feels to have reality suddenly drop away beneath you like a trap door.

Jackson's performance, which has already won the International Emmy and a BAFTA award, elevates the film. With her trudging walk and features that veer between slack confusion and ferocious certainty — flinty impatience is a Jackson trademark — her Maud has none of the decorous radiance that Julie Christie and Julianne Moore brought when they were playing characters with dementia. Instead, like Anthony Hopkins' scorching turn in the upcoming film The Father, the 84-year-old actor gives us a lost soul who, in trying to stay afloat in reality, becomes an exhausting, sometimes cruel burden. Both victim and victimizer, Maud hurts her daughter and everyone else who loves her.

What stops the show from being punishing is that Maud's dementia isn't the only story. Elizabeth Is Missing doesn't wallow in misery. Approaching dementia on the angle, it lets us see Maud's unlikely heroism as she struggles against her own declining faculties to find out the truth. It keeps us guessing about the fate of the missing women who haunt Maud's mind, and it makes us wonder if everything will tie together in the end.

The whole story does tie together — in truth, far too neatly. Yet I didn't mind, for such closure feels positively merciful. You see, the great thing about a mystery story is that you can solve its puzzle. With dementia, there's no solution.

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