As good as “The Bletchley Circle” was the first time around, there was reason to worry that a second adventure would risk stretching this clever little premise beyond its expiration date. Those fears are at least partly realized by season two, which not only reassembles its team of female World War II code-breakers to solve a new mystery, but also bifurcates the four-episode run, raising the question of how much trouble these now-ordinary gals can keep credibly stumbling into. Still fun on its own terms, the encore takes an unexpected little gem and transforms it into “Murder, She (and She and She and She) Wrote.”

The two-case format essentially follows a template used on “Luther,” but that show has the benefit of focusing on a world-weary detective who has an excuse for running into serial killers.

Here, the story begins promisingly enough, as the four central women are spurred back into action when a former colleague (Hattie Morahan) from their code-breaking days winds up on trial for murder. She’s accused of killing a scientist with whom she’d worked and had an affair during the war, and she refuses to disclose why she’s reluctant to provide information that might exonerate her.

The narrative rather quickly turns to a fairly obvious plot that has to do with chemical-weapon experiments, as the four, spurred on by Jean (Julie Graham) but ostensibly led by married wallflower Susan (Anna Maxwell Martin), unearth clues against a Cold War backdrop.

Yet with that mystery resolved in the second hour, attention turns to a second conspiracy, at which point Susan takes her leave, and it starts to feel like this latest “Circle” is doing little more than spinning plates.

“Bletchley” caps off a night of 20th-century PBS drama that includes additional seasons of “Mr. Selfridge” and “Call the Midwife,” but it’s gone from perhaps the most enjoyable of that trio to the one least suited to a return engagement.

And it’s an unfortunate endorsement of what the male authorities around the intrepid “Bletchley” alums keep telling them — namely, that even with those mad code-breaking skills, it’s important to know when to leave well enough alone.

TV Review: ‘The Bletchley Circle,’ Season 2

(Limited series; PBS, Sun. April 13, 10 p.m.)

  • Production: Produced by World Prods. in association with Content TV for ITV.
  • Crew: Executive producers, Jake Lushington, Greg Phillips, Geoff Webb; producer, Trevor Hopkins; director, Jamie Payne; writer, Guy Burt; camera, Adam Suschitzky; production designer, Claire Kenny; editor, St. John O’Rorke; music, Nick Green; casting, Rebecca Wright. 45 MIN.
  • Cast: Anna Maxwell Martin, Julie Graham, Rachael Stirling, Sophie Rundle, Hattie Morahan, Faye Marsay
  • Music By: