All you really need to know about “Their Finest” is that Bill Nighy is in it. Bill (both “Marigold Hotels” and “Love Actually”) could save “Hamlet” with Donald Trump and Kellyanne Conway. He is the ultimate British light comedy actor who can step into a serious, troubled MI6 agent on a moment’s notice (“The Worricker Trilogy”).

But in “Their Finest,” a movie within a movie taken from the book by Lissa Evans (“Their Finest Hour and a Half”) and directed by Lone Scherfig, Bill is given serious competition by a cast of superb British actors: Gemma Arterton, Sam Claflin in the leads, with back up pros like Helen McCrory and Jack Huston.

Of course you need to know that “Finest” takes place at probably the lowest point in Britain’s battle with Germany in 1940, when the Brits were getting the crumpets kicked out of them by the Luftwaffe.

In the center of this, the British film industry, which up to the war had been producing light romantic comedies, is set to work making propaganda films to boost morale.

The filmmakers come up with a heart-tugging back story set in the middle of the heartbreaking evacuation of thousands of British soldiers stranded at Dunkirk.

Unable to make a big screen war movie, what with all the young actors fighting for their lives in real battles, they come upon a newspaper story about two shaky sisters who took their alcoholic father’s wobbly boat and joined the hundreds of other boats of all kinds and their courageous handlers headed to Dunkirk to rescue their children.

“Finest” takes place on the back lots of the crumbling film studios in London, where a huge tank has been set up to film the water scenes.

In the smoky back rooms around the corner, writers start pumping out the scripts. Of course they’re guys. Who else but alpha scribes could write a war film?

But the front office needs romance, which of course requires a “woman’s dialogue,” which is above the pay grade of the male writers, or so the Home Office thinks.

Thus lovely Catrin Cole, an advertising copywriter facing the end of a frayed romance with a crippled painter she’s been supporting (Jack Huston), is thrown into the lions’ den with, wouldn’t you know, a bright, cynical and handsome lion — Tom Buckley (Sam Claflin).

Together, against stiff resistance from the British Ministry of Information and their stiff upper lip watch dogs, the young couple, desperately resisting the inevitable bumping of hearts, come up with a great script.

Now they need an actor to play the corpse of the two girls’ drunken father. Enter Bill Nighy, who refuses to play a dead hero and works his way into enhancing his role, which will eventually raise the “dead” father to a starring part. It’s Bill’s finest part since his sodden rock star in “Love Actually.”

In case you think this is all war-time light-hearted romance and comedy, bite your tongue.

“Finest” is so much more than that. There is heartbreak here in the rubble full of dead Brits and in the writing room and on the set. Just when you think you know what’s going to happen in the final shots, you’ll discover that there are several final shots, some bursting with laughter, others soaked with tears and superb acting, sparkling and edgy writing, all keeping us thinking and waiting.

Midway in the film, the actors and crew, a disparate crowd of strangers, none of whom know if they will have a tomorrow, begin to meld and grow to become brothers and sisters, friends and lovers on a mission that in the midst of London’s firefight is no less dangerous than the front. Prepare to stand up and applaud at least six times.

Sebastian Blenkov, the film’s cinematographer, shoots the scenes in a nostalgic blend of caramel and smoke, evoking the early days of Britain’s Michael Powell (“Thief of Baghdad”) and the famed Rank Studios.

“Their Finest” ranks with some of the finest of British films.

J.P. Devine is a former stage and screen actor.

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