Matthew Rhys as Darcy in “Death Comes to Pemberly.” (Courtesy of Robert Viglasky/Origin Pictures 2013 for MASTERPIECE/Courtesy of Robert Viglasky/Origin Pictures 2013 for MASTERPIECE)

“Death Comes to Pemberley,” a lovely but languorous two-episode drama beginning Sunday night on PBS’s “Masterpiece Mystery!” series, would seem like a wish come true for the most devout public-television freak: It combines the greatest of all literary period romances with the casual thrills of a murder investigation.

Based on P.D. James’s best-selling 2011 novel, the story grafts a whodunit onto the happily-ever-afterglow of Fitzwilliam (“Mr.”) Darcy and Elizabeth Bennet, the now-married lovers immortalized in Jane Austen’s 1813 classic “Pride and Prejudice.” I think we can all agree that it’s far too late to get uppity about mucking around with that literary treasure — the characters have undergone every indignity and adaptation imaginable, up to and including a zombie attack — but it’s understandable to greet “Death Comes to Pemberley” with a measure of friendly suspicion.

Set six years after Austen’s tale, “Death Comes to Pemberley” finds the Darcys and their young son living in relative bliss at Pemberley, his sprawling family estate.

While preparing for a party, their world is upended by news of a murder in the nearby woods. Mr. Darcy (Matthew Rhys of “The Americans”) and other men rush out to investigate. Elizabeth (Anna Maxwell Martin) isn’t at all that surprised to learn the commotion involves her vain little sister Lydia (“Doctor Who’s” Jenna Coleman) and Lydia’s no-good husband George Wickham (“The Good Wife’s” Matthew Goode).

The murder victim is one Capt. Denny (Tom Canton), a friend of Wickham who was riding toward Pemberley in a stagecoach with Wickham and Lydia. After an argument, Denny apparently jumped out into the woods; Wickham went after him and Denny wound up dead.

With Wickham facing murder charges and Mr. Darcy stressing out over defending both his lifelong frenemy and the reputation of the Pemberley estate, it’s up to Elizabeth, of course, to tie on her sleuth bonnet and try to get the bottom of what really happened.

At nearly three hours long, “Death Comes to Pemberley” runs around in a few too many circles. The crime (and the crime solving) can’t hold a candle to the delight of watching Elizabeth and Mr. Darcy drift apart and then back together in a refrain of their story of obstinate love — a task Rhys and Maxwell Martin acquit themselves of quite well.

Masterpiece Mystery!: Death Comes to Pemberley

(90 minutes) begins Sunday at 9 p.m. on WETA and MPT. Concludes Sunday, Nov. 2, at 9 p.m.