Lost and confused in a world of her own making, one mistaken recollection after another. Generating sympathy for the iconoclast of 20th century Britain is no mean feat, particularly with a story so hamstrung by its refusal to expand upon the very policies that made its subject worthwhile. That The Iron Lady is elevated even marginally beyond historical summary is down almost entirely to Meryl Streep, who is as convincing as the younger Thatcher as she is the less familiar one: the elderly woman, reduced to pottering around her flat, with only a carer and dead husband to mind.

Dennis Thatcher is Jim Broadbent in all but name. As an actor, he has become increasingly adept at being himself. Broadbent’s natural tendency towards grandfatherly wisdom serves him just as well here as it did portrayals of Horace Slughorn and Dad. Was he ever young? Dennis himself died a number of years ago, sometime between his wife losing her grip on power and losing it on everything else. He returns in spirit to accompany her through the milestones of her day: an early breakfast, perhaps, or the indulgence of afternoon tea. Sometimes he lies next to her in bed, crossword in hand, until she suddenly snaps to. Then there is only silence.

These are glimpses into a confused world, familiar to anyone with experience of dementia. It is a particularly cruel punishment. Few who have watched from its periphery will forget its ravages, or the shell left behind. When we think of growing old, we often have a misty-eyed notion of our mind and body leaving in unison, but fate is rarely so kind. More often, your body fails, or your mind goes first and that’s somehow even crueller. That it should happen to a woman so celebrated for her intellectual vigour is what some opponents might consider justice. They are wrong.

The Iron Lady is an attempt to bridge time, with Margaret Thatcher as its conduit. Streams swirl together in a reassuring collage, until there is no distance between the then and the now, the here and the gone. Only in her fleeting moments of clarity does the former Prime Minster come to face the enormity of what has been lost; the humbling knowledge that her life’s thread is now fraying, leaving a once fearless leader awash in a strange sea. And yet, to witness her sometime lucidity – the still-sharp burst of strident polemic at a polite dinner party, for instance – is to fall for her, just a little. I suppose you could say she’s awfully convincing at being awfully cruel.

In dealing with the confused, some find kindness in simply playing along. Dennis is not one of them. Even in spirit, he riles against the replaying of past glories, or the notion of medical intervention. His purpose is clear. He cannot leave until his wife accepts her fate. In that, he is effective. The problem, then, is not Broadbent or Streep, but the fatally limited scope of their story. It’s incredible to think that one couple played host to the rise and fall of an entire underclass. Riots, strikes, dissident terrorist groups and a political party revealing its own cancerous underbelly; such a rich history beneath their feet, yet the screenplay waves it all away. Any ramifications belong almost entirely to Thatcher. It is her demise we see, and even then from afar. It’s like holding court with Marie Antoinette, and neglecting to mention why she’s always prattling on about cake.

Turned against compromise by an embattled stint in the backbenches; by a blustering father and, yes, by virtue of being a lone woman in the male chamber of politics. The path that made Margaret Thatcher all she was – good and bad – is largely absent. Instead, we have a scene of annunciation pilfered from The Kings Speech (likewise its dusty histrionics), and occasional disagreements with cabinet colleagues. No central performance, no matter how blustery, can overcome such a beige palette. Her opponents will undoubtedly label it a whitewash, while supporters will condemn the portrayal of their hero in such an advanced state of decline. They might take comfort that she is at least offered some sympathy, which is considerably more than can be said her targets.

Margaret Thatcher wanted her life to matter. She considered there to be no fate worse than dying while doing the washing up. Success: her life did matter. Perhaps not in the way she might have intended, but it mattered all the same. All that remains now is the washing up.