WHAT IT’S ABOUT This movie dramatization of Jonathan Smith’s 2015 novel, “The Churchill Secret: KBO,” charts a dramatic incident in the most dramatic of lives, a devastating stroke that felled Prime Minister Winston Churchill (Michael Gambon) for a month. His family — including his beloved wife, “Clemmie” (Lindsay Duncan), and adult children, Diana (Tara Fitzgerald), Randolph (Matthew Macfadyen), Sarah (Rachael Stirling) and Mary (Daisy Lewis) — gather for the deathwatch. Both the liquor and emotions flow. But it is a nurse, Millie Appleyard (Romola Garai), who keeps her head, and saves the day.

MY SAY Churchill was one of the most dynamic individuals of the 20th century, until June 23, 1953, when — between a speech and a toast — he suffered a stroke. A cover-up ensued, the compliant press agreed to the ruse, and a postwar powerhouse was effectively without a leader while he recovered. That sounds like a good story, except that’s not quite the one “Churchill’s Secret” — directed by Charles Sturridge (“Shackleton”) — wants to tell. The camera pulls you inside the grim interior of the family home where a great leader lies prostrate, and his children squabble over gin martinis and how this setback has inconvenienced them. For the most part, there the camera lingers, in the half-light and shadows.

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“Churchill’s Secret” is beautifully crafted and acted — with this crew, how could it not be? — but you’ll be left the impression that it’s also confined and cramped. There’s a reason for that. “Churchill’s Secret” is an interior exploration of the most public of men, at a moment when his death seemed likely, if not inevitable. Gambon’s Churchill slowly battles his way back to health — “keep buggering on” his rallying cry — and “Secret” pulls you back to that story it really wants to tell, which is the state of his mind during convalescence. He had his own cryptic “Rosebud” obsession, an old Tin Pan Alley song, “I’m Forever Blowing Bubbles.” He also has a favorite poem, the William Ernest Henley classic, “Invictus” (“Unconquerable”) which he recited with Millie. He has recurrent visions of a child in a frock — his beloved daughter Marigold, born just days after World War I ended in 1918. Marigold — whom a doting father had once called “the Duckadilly” — would die three years later.

Churchill’s “secret,” as it turns out, wasn’t just the one he kept from the nation and the world about his health. It was the secret he kept from himself — of a cherished daughter and his guilt and grief over her death. He recovered his health, and ended his second term as prime minister two years later. His secrets — both of them — were preserved.

BOTTOM LINE: Nicely crafted, and Gambon — as always — is superb, but this “Masterpiece” movie can also be turgid and lugubrious.