In the days before the Normandy landing, British prime minister Winston Churchill retrieves his hat along a blustery coastline before uttering some of the worst dialogue you'll hear this summer: "Beaches always bring it back ... Almost 30 years ago now ... So many young men."

Winnie promptly returns to No 10, is introduced to the new secretary (Ella Purnell) – whom he will alternately bawl out and charm for the rest of the movie – dons his special breeches and heads to the last briefing on Operation Overlord. There, a gathering of relevant historical figures – Bernard Montgomery (Julian Wadham), King George VI (James Purefoy), and Gen Dwight D Eisenhower (John Slattery) – are helpfully informed that "This is our last briefing on Operation Overlord, the assault on the German occupying forces in the north of France. "

We've seen a cathedral of decent Churchills on screen over the years, often with footnotes of petulance (see Brendan Gleeson in The Gathering Storm) and pettiness (John Lithgow in The Crown). But he has never looked quite as shambolic as he does in this new film, directed by Jonathan Teplitzky from a script by Alex von Tunzelmann. Brian Cox does excellent work, often against the script. His Churchill huffs and puffs against the D-Day landings, partly because he recalls the death toll at Gallipoli during the first World War, and mostly because he feels left out.

(Spoiler alert: It’s true that Churchill expressed reservations about the Normandy beach landing, but there’s no evidence that he attempted to widen the cross-Channel invasion. Had he insisted, he could not have been over-ruled by the Americans, as he is here. Second spoiler alert: there was a second World War.)

Director and writer attempt to humanise their subject by showing him rehearsing his speeches and standing around in his underpants. (As with Batman’s utility belt, Churchill only gains his superpowers when he puts his hat on.)

Unfortunately, the film's attempt to conflate Churchill's "black dog" depression and D-Day is not just historically questionable, it dramatically doesn't ring true. Cox responds with a masterclass, regardless. And there are some beautifully written supporting characters enlivened by astute performances, particularly from James Purefoy as King George and Richard Durden as South African prime minister Jan Smuts.