An occupied British island, plucky locals “resisting” the Nazis with a fake “book club,” hiding livestock and a recipe for using the one vegetable they were allowed in creative, whimsical ways  — “The Guernsey Literary & Potato Peel Society” has “Whisky Galore!” written all over it.

But that’s not the movie Mike Newell (“Four Weddings and a Funeral,” “Enchanted April”) got out of this British novel. It’s not his fault the tale turns, unexpectedly, towards “The Sorrow and the Pity” and winds up in melodrama-land.

Not totally his fault, in any event. A two hour movie wrung out of a 100 minute novel? Yeah, that’s on Mr. “Harry Potter and the Goblet of Fire” (one of the better Potters, I thought).

“The Guernsey Literary & Potato Peel Pie Society” was born in desperation, at the spur of the moment. One of the British “Channel Islands,” Guernsey was invaded by the Germans right after France fell. It opens with staggering locals facing Nazi occupiers in the dark of night. No drunken declaration of “This is OUR island, not theirs,” can save them after curfew. But the whopper, “We have a book club,” does. “The Guernsey Literary & Potato Peel Pie Society” is born.

It’s a story the comic adventure novelist Juliet Ashton (Lily James of “Downton You-Know-What”) hears after the war, in a fan letter of sorts.

She and her publisher (Matthew Goode, perfect) are pondering the cash and notoriety her “Izzy Bickerstaff” books give her. Critical acclaim, gravitas and intellectual satisfaction? Not so much.

The war has left her dissatisfied and a little shell-shocked and shaky. Free-spending GIs are still in town, the nightclubs are swinging again — the ones not still in the ruins of the Blitz. “Don’t Waste Bread: Others Need It” is still stamped on every letter.

Maybe the guilt kicks in with that fan letter from a quirky book club, somebody who clung to an early book she wrote during long years of German occupation, when the mail, decent food and contact with the outside world was cut off.

The letter, from a farmer named Dawsey Adams (Michael Huisman), read in voice-over narration, is nostalgic (this is 1946), mysterious and just a little romantic. Juliet is intrigued. She must know the inspirations for this “club” of readers on a German-occupied island where life was put on hold for five years.

“Why did a roast pig have to be kept a secret? How could a pig cause you to form a literary society? And most important of all, ‘What is a potato peel pie?”

Juliet had a rather easy war of it, all things considered, even though everybody suffered loss. And now? She’s got a handsome American diplomat beau (Glen Powell) and the celebratory posh life is all around her. Guilty conscience?

Dawsey writes back, “The Germans took all our animals away. It was against the law to keep even one.” They were ordered to simply grow potatoes instead. “A proper meal was only had in memory, like the post, which they had suspended, the radio, which they had taken,  and the telegraph cables which they had cut.”

The pie? It was invented by the postmaster (Tom Courtenay) as a way of making something of the one thing the Huns let them keep — potato peels.

He writes of their “club” and Juliet takes the bait. She must venture to 1946 Guernsey, forsake her American beau and find this flinty Mrs. Maugery (testy-grim Penelope Wilton, another “Downton” alumna), this Isla (Katherine Parkinson), who distilled homemade gin.

On picturesque Guernsey, isle of rocky cliffs, seaside forests, small farms, cozy cottages, bicycles and horse-drawn carts, she sees a cute story for the London Times. But the book “society?” They’re adorable on first meeting. And less cooperative on the second.

It’s not all nostalgia, grins and giggles. The war is still a recent horror to these quaint locals. Rifts, feuds and bitterness, with memories of evacuated children, missing locals “transported” off by the Germans and hardship stick with them.

Any film fan knows where this is going, but the source novel takes pains to trip up expectations as Juliet digs into the “real story,” finds the “real” objections to her telling it and gets sidetracked from the future that seemed to lay itself before her as she embarked on this journey.

James is a winsome presence, with able support from Huisman, Powell, Goode, Courtenay, Parkinson and Wilton.

But the story veers into pure melodrama, a tale of missing persons, grudges, babies whose real fathers we aren’t sure about and a quite young novelist trying to “investigate” and find the true story — from the club members, or from her unsentimental on-island landlady (Bronagh Gallagher of “The Commitments”).

As we know how this is going to pan out, it’s a puzzlement how Newell wrings 30 unnecessary minutes out of the journey. Guernsey is a great setting, James a properly plucky English heroine and her potential suitors nicely contrasted.

The most intriguing thing here is the “collaborationist” angle, something usually only seen in stories of Occupied France.

But that’s not enough to make “The Guernsey Literary & Potato Peel Pie Society” warrant its long title, its drawn-out ending or patience-testing, flaccid running time.

MPAA Rating: TV-14

Cast: Lily James, Glen Powell, Matthew Goode, Jessica Brown Findlay, Tom Courtenay, Penelope Wilton

Credits:Directed by Mike Newell, script by Don Roos, Kevin Hood and Thomas Bezucha, based on the novel by Mary Ann Shaffer and Annie Barrows. A Studio Canal/Netflix release.

Running time: 2:04