Let the record show that Agatha Christie’s “Witness for the Prosecution” began as a short story you can read in 30 minutes. The famous Billy Wilder film (1957) was a two-hour affair; a 1982 TV version was a bit more compact. Yet somehow at the Olney Theatre Center, “Witness for the Prosecution” is taking three hours.

A three-hour play is no crime. It is overkill, though, for a mystery that keeps running back and forth over the same ground.

The plot, in short: Leonard Vole, young and handsome, is suspected of murdering a kindly old lady who found him appealing. Vole is poor, and she had money. Turns out she left her dough to him.

Vole’s alibi is his wife, Romaine. She’s German. She’s shady. How will she testify? Who’s more trustworthy — husband or wife? It’s he said, she said — a lot of talk but not much drama.

Director John Going is back leading what could be called the Olney’s Whodunit unit — veterans of Christie’s “The Mousetrap” (a stylish success a few seasons ago) and “Night Must Fall” (another murder mystery chestnut). Liz Covey once again provides understated period costumes, this time including wigs for the huffy barristers. And James Wolk again supplies sunless British interiors, with the star here being a heavy wood-paneled rendition of London’s Old Bailey criminal court.

That set draws applause as it glides downstage, partly because it looks so right — but also perhaps because it’s one of the few things that actually happens onstage. The show is comfortably old-fashioned, but the stuck-in-place story lacks the colorful characters and intriguing bustle that made “The Mousetrap” such lively fun.

Here you’re left watching capable actors pacing around and stroking their chins as their characters rehash the sketchy evidence. The cast, luckily, is about as good at this as it can be. Jeffries Thaiss gently doles out the charm as Vole, the chief suspect. Bob Ari, James Slaughter and Alan Wade harrumph expertly as the various barristers. As Romaine, Andrea Cirie radiates menace; she even manages not to sound silly with a thick accent that more than once renders “Nothing but the truth” as “Nussink but ze truce.”

The record should also reflect that “Witness for the Prosecution” endures because of a surprise ending that plainly energized much of Saturday night’s audience. But even this sudden twisting finish is over-padded, and not delightful enough to exonerate the courtroom drama’s meandering.

Pressley is a freelance writer.

Witness for the Prosecution

By Agatha Christie. Directed by John Going. Lights, Dennis Parichy; sound design, Matt Rowe. With Jim Scopeletis, Paul Morella, R. Scott Williams, Monica Lejewski, Carolyn Myers, Joe Palka, Drew Kopas, Alan Hoffman, Jenny Donovan, DC Cathro and Greg Twomey. About three hours. Through Oct. 26 at the Olney Theatre Center, 2001 Olney-Sandy Spring Rd., Olney. Call 301-924-3400 or visit olneytheatre.org.