Fortuitously timed to provide an additional dose of World War I-era melodrama for those bereft over “Downton Abbey” finishing its season, “Parade’s End,” a five-hour miniseries HBO will air over three successive nights, feels grittier than that acclaimed PBS standout, but in terms of relative merit, is a much weaker tea. Benedict Cumberbatch (“Sherlock,” the upcoming “Star Trek”) leads a splendidly assembled cast, but his emotionally stunted character and uncomfortable circumstances make this stiffest-of-upper-lipped love stories a muddy slog (sometimes literally) through war-tinged romance, awash in duty and longing.

Even the scheduling suggests HBO wasn’t entirely sure what to expect from this stately but slow-going exercise, slotting it in a way that could diminish its exposure.

Adapted from Ford Madox Ford’s novels by Tom Stoppard, the story hinges on Cumberbatch’s Christopher Tietjens, a patrician government statistician so officious he corrects the encyclopedia in the margins.

As the story opens, Tietjens is uncomfortably headed to the altar with Sylvia (Rebecca Hall). Yes, she’s beautiful and alluring enough to make even smart and powerful men behave stupidly, but she’s also pregnant with a child he can’t be sure is his or that of her married lover.

Sylvia’s subsequent extramarital dalliance drives a pronounced wedge between them, but Tietjens’ deeply ingrained sense of honor rules out divorce. He’s willing to stoically endure until he encounters a winsome suffragette, Valentine (Adelaide Clemens), instantly falling for her, yet reluctant to act on those feelings.

World War I eventually intervenes, with Tietjens lamenting its uncivilizing effects on the way of life the aristocracy had known and cultivated. “We’re all barbarians now,” he says.

Except Sylvia, for reasons a little unclear, decides she wants to repair and save her marriage, only to be rebuffed by her husband, who seems oblivious to her efforts.

Directed by Susanna White, “Parade’s End” (a reference to the waning commitment to keep up appearances and avoid “untidy” lives) features an impressive array of actors in smaller roles. They include Janet McTeer and Miranda Richardson as Sylvia’s and Valentine’s mothers, respectively; Stephen Graham (“Boardwalk Empire”) and Rupert Everett as Tietjens’ best friend and his brother; and Rufus Sewell as a crazy clergyman, prone to saying outlandish things.

Ultimately, though, the story boils down to its central love triangle, with the sides stretching out a little too long as viewers wait for Tietjens to return home and choose whether to pursue happiness and risk public humiliation, or remain in his shattered and unhappy marriage.

Beyond boasting one of the best names in showbiz, Cumberbatch conveys intelligence and strained intensity like nobody’s business, and Hall is equally terrific as the Jessica Rabbit-like Sylvia, who isn’t necessarily bad; she’s just drawn that way.

For all that, watching the five-plus hours at times feels like its own version of trench warfare, inching from one small milestone to the next while waiting for something of consequence to happen.

Perhaps that’s why by the time it’s over, “Parade’s End,” with its beautiful trappings and promotable cast, begins to mirror Tietjens’ marital predicament — looking better from afar than it does up close.

Camera, Mike Eley; production designer, Martin Childs; editor, Kristina Hetherington; music, Dirk Brosse; casting, Karen Lindsay-Stewart. 125 MIN.

Contact Brian Lowry at brian.lowry@variety.com

(Miniseries — HBO, Tues. Feb. 26, 9 p.m.)

Cast: Benedict Cumberbatch, Rebecca Hall, Adelaide Clemens, Roger Allam, Anne-Marie Duff, Rupert Everett, Stephen Graham, Janet McTeer, Miranda Richardson, Tom Mison, Malcolm Sinclair, Tim McMullan, Rufus Sewell.

Filmed in the U.K. and Belgium by Mammoth Screen in association with Trademark Films, BBC Worldwide and Lookout Point; co-produced by BNP Paribus Fortis Film Fund and Anchorage Entertainment. Executive producers, Michele Buck, Damien Timmer, Ben Donald, Simon Vaughn, Judith Louis, Tom Stoppard; producers, David Parfitt, Selwyn Roberts; director, Susanna White; writer, Stoppard, based on the novels by Ford Madox Ford.

Camera, Mike Eley; production designer, Martin Childs; editor, Kristina Hetherington; music, Dirk Brosse; casting, Karen Lindsay-Stewart. 125 MIN.