No need to call the cops – Harry Styles can act. A queer period drama requires a reliable, watchable cast and, apart from the odd stilted delivery, the actor-singer does the job well. He also looks the part, but no surprises there.

I mention Styles upfront because a third of the audience for this Adelaide Film Festival Special Presentation Australian Premiere were young girls at their first film festival only to see Harry Styles. What they made of this sad story is hard to know, although inappropriate giggling offered a hint.

This is a slow ensemble piece that shows what a ruinous tragedy anti-gay laws were for generations of men and women. Styles plays Tom, a police constable in 1950s Britain who wants to be straight but whose hopes are dashed when he meets Patrick (David Dawson), an older man reconciled to his homosexuality who lives a double life working at a museum.

Tom is already seeing Marion (Emma Corrin) in a lukewarm and conventional courtship, encouraged by his seniors at work who told him marriage was a prerequisite for advancement. She adores him and he is fond of her, but in secret he lets his guard drop and explores real passion with Patrick.

The pace is rather plodding as we switch too often between time zones 40 years apart and the older and younger versions of the trio of Tom, Marion and Patrick, through the odd device of Patrick’s diary. What 1950s homosexual in their right mind would have left such a detailed diary?

My Policeman begins in England in 1999, in a small seaside town where a middle-aged Marion (Gina McKee) has taken in from hospital the older Patrick (an unrecognisable Rupert Everett) following a stroke. She doesn’t want him sent to a nursing home and sets up a room for him, against the wishes of husband Tom (Linus Roache). It is not a happy home: Marion and Tom are miserable, and Patrick can barely speak – when he does, all he wants is Tom, and a cigarette.

The toll on all of them becomes evident as we go back in time to watch the truth of her marriage slowly dawning on the younger Marion. Tom, a simple man, believes that if you don’t talk about, it never happened.

The film is lovely to look at and has an atmosphere of drawn curtains, particularly in Patrick’s lamp-lit, art-filled rooms in Brighton, where it all began. Directed by Michael Grandage, it was based on a novel whose story was inspired by the life of English novelist EM Forster, the author of A Room with a View and Howard’s End and a closeted homosexual whose lovers included a policeman.

While superficially concerned with an unravelled love triangle, the film digs deeper as Marion and Tom wrestle with the constraints that society imposed on them. Only the outsider, Patrick, knew who he was.

My Policeman had its Australian premiere as part of the 2022 Adelaide Film Festival, which runs until October 30. The film will have a general release across Australia next month.

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