‘Sherlock’ picks up where it left off

Fourth season comes with mystery, murder and a baby

London — There’s something rotten in Baker St.

It may just be the diaper of baby Rosamund Watson, whose arrival at the start of the fourth season of “Sherlock” disrupts the relationship between brilliant, demanding detective Sherlock Holmes and his long-suffering friend Dr. John Watson.

The offspring of John and his enigmatic wife Mary — with Sherlock as a somewhat skeptical godfather — is one of the few things program-makers are willing to reveal before the first of three new episodes airs Sunday on the BBC in Britain and on PBS in the U.S.

“The unofficial alternative title for episode one is ‘The Three Watsons,’ because the baby changes the dynamic,” said Mark Gatiss, the show’s co-creator. But, he stresses: “Not in a cutesy way.”

Fears of domestic coziness are quickly banished in the opening episode, officially titled “The Six Thatchers.” Its central whodunit involves half a dozen plaster busts of former Prime Minister Margaret “Iron Lady” Thatcher.

That’s just the starting point for 90 minutes of virtuosic puzzle-solving, prickly friendship and the spectacular return of ghosts from one character’s past.

The new episodes pick up where the previous season left off almost three years ago. Since then, fans have had to make do with a one-time special last year that took the characters back to their Victorian roots.

The show’s success means Benedict Cumberbatch, who plays Holmes, and his Watson, Martin Freeman, are in huge demand, and “Sherlock” must fit around their other projects. Cumberbatch jumped back into Holmes’ deerstalker hat right after finishing work on the Marvel adventure “Doctor Strange.”

Everyone involved in “Sherlock” is sworn to spoiler-free secrecy about the plot, although Cumberbatch offers that it finds the great detective “at the top of his game, at the very bottom of his soul.”

He says viewers will see more of Sherlock’s vulnerabilities as he faces crises both personal and professional.

“It would be very dull if he was permanently a kind of impregnable hero who was always right,” Cumberbatch said during a break from filming. “There’s some spectacular own-goals, misses and mistakes in this series.”