The later films of David Lean are works of such visual ambition and scale that they can let the viewer lose track of the connective thread, the relationships and characters that make “Bridge on the River Kwai,” “Lawrence of Arabia” and “A Passage to India” so compelling.

We see the editor-turned director, the craftsman and painter of grand cinematic landscapes, but forget the emotional triangles that are the building blocks of even his literary adaptations or biographical epics.

“The Passionate Friends” (1949) is a simple love triangle, another “Brief Encounter” (1945) melodrama about infidelity that here isn’t just considered, but consummated. This time, the third party in the love triangle is much more present.

It’s not “wartime rationing” prosaic, sad and drably middle class like “Encounter,” but posh, even if it feels very much hemmed-in and soundstage-bound until it opens up in an extravagant, lavishly-photographed (in black and white) third act, set and shot largely in the French Alps.

Following Lean’s classic Dickens adaptations “Great Expectations” and “Oliver Twist,” this return to melodrama was a box office flop and tempted Lean into imitating Hitchcock (“Madeleine”), even trying his hand at period piece domestic comedy (“Hobson’s Choice”) as he slowly re-acquired the prestige and box office success that allowed him to make motion picture “events” from “Kwai” until the end of his days.

“Passionate Friends,” based on a novel by “free love” sci-fi pioneer H.G. Wells, is a post-war tale consumed by a long flashback to how our lovers were connected before World War II, and otherwise barely mentions the conflict, loss and privation that Britain was still crawling out of in the late ’40s.

Ann Todd is our heroine and narrator, Mary, a great beauty with an idea of “the sort of life” she wants — rich and privileged. When we meet her, she’s narrating her way on her first post-war holiday, flying to Switzerland (Haute-Savoie, France, actually) to the lakeside Hotel Splendide for a little pampering.

Her husband is stuck in London on business, but his secretary (Betty Ann Davies) accompanies her and makes everything go effortlessly as she awaits the man who pays for all this comfort and high-fashion.

But an old flame checks into the room next door. And speaking from the fictive present — after she’s realized who is staying there — she recalls her great pre-war love affair with academic biologist Steven (Trevor Howard of “Brief Encounter”), the man she told “I shall never love anyone as much as I love you.”

The lengthy flashback isn’t the most graceful one Lean ever offered, but basically Mary and Steven stumble into each other on New Year’s Eve, 1939, after they’d broken up and she’d gone on to marry money. They begin to see each other as “friends,” which her older husband, Howard, tolerates. Howard is played by forever-cuckolded Claude Raines, who made more than one film where he’s the rich, older spouse whose wife is tempted away.

The “passionate friends” revert to being more than friends over the course of this long flashback, and even decide to tell Howard and make a go out of being together. But that didn’t work out, and now nine years later, they’re spouse-free in Switzerland and about to renew their acquaintance in one of the most striking settings on Earth.

Todd is the heart and soul of the picture, and her performance lets us see the inner turmoil of a woman who wants to have her Chanel and Switzerland, and her great love, too. She is at her best in scenes where Mary recognizes the dilemma she’s put all involved in and grows frantic — in that reserved and ever-so-English way — about what to do.

Rains makes Howard dashing, deliberate and distracted enough to let all this play out, but determined to bring down the wrath of Howard when it all blows up.

Trevor Howard is as passive here as he seemed in “Brief Encounter,” a near innocent who can’t quite resist whatever Mary is pulling him back into or see the risks of following one’s heart…or impulses.

The picture’s three-hander structure limits its scope, with much of its running time consumed with drawing room conversations, close-ups of each character in her or his emotional distress, and the clumsy way Mary and Steven handle their indiscretion and keeping it secret.

A rich man, humiliated, can be a dangerous thing.

Lean gets things moving and scenic in the third act, seamlessly blending location exteriors with Pinewood Studios sets, rear-projections and the like.

And the finale manages some suspense, even if it feels like the cop-out many a melodrama of that era leaned into for its ressolution.

But Lean completists will take pleasure in the connective tissue that binds “The Passionate Friends,” a lesser Lean film, with his other work, get a sense of his first serious dalliance into an epic setting and enjoy one last passive romantic turn by Trevor Howard, before a career of grumpy authority figures and military men would all but erase the romantic Lean first saw in him.

Rating: TV-PG, infidelity, smoking

Cast: Ann Todd, Trevor Howard and Claude Rains, with Betty Ann Davies, Isabel Dean and Wilfred Hyde-White.

Credits: Directed by David Lean, scripted by Eric Ambler and David Lean, based on the novel by H.G. Wells. A Univeral/General Film Distributors release, a J. Arthur Rank production restored and now on Tubi, Amazon, etc.

Running time: 1:35