Kenneth Branagh tells an amusing story of how he met the late author Henning Mankell, the author of the “Wallander” novels that are the basis of the PBS mystery series.

In 2007, the Ingmar Bergman Festival in Fårö, Sweden, was screening Branagh’s version of the Mozart opera “The Magic Flute,” a work that Bergman had himself filmed some three decades before.

“It was a small cinema. It was a long film, and there was a huge queue for the loo,” remembers Branagh, using the British term for the bathroom. So Branagh took matters into his own hands, stepping outside behind the back of the theater to relieve himself.

As he was doing so, he looked over and saw the author Mankell next to him doing the same thing. The two introduced themselves, before going back inside to wash their hands before shaking them.

Over the next couple of days, they talked. “He showed me around Bergman territory a bit,” says Branagh, referring to the landscape where the iconic director shot many of his films. (Mankell was married to one of Bergman’s daughters.) “As a result, we talked about Swedish drama and programs’ influence on him, and I got a sense of the man.”

Eventually, they discussed Branagh playing Wallander, and the two became good friends. After Mankell died at 67 of cancer in October, Branagh wrote a loving tribute to him in Britain’s Guardian newspaper.

The fourth and final season of “Wallander” begins airing Sunday. Like the previous seasons, it is only three episodes long. Because of the time it takes to produce each as well as the multitalented Branagh’s work schedule, it has taken eight years to do the 12 stories that will comprise the series.

The actor is currently starring in a London West End production of the comedy “The Painkiller.” Later in the summer, he directs Lily James, “Game of Thrones’?”Richard Madden and Derek Jacobi in a staging of “Romeo and Juliet,” and then he will trod the boards again himself with John Hurt this August in John Osbourne’s “The Entertainer.” After that, he’s presumably off to star in Christopher Nolan’s World War II epic “Dunkirk.”

The actor is an executive producer on the series as well as playing Kurt Wallander, a melancholic police inspector based in the southern part of Sweden. He’s divorced, has a grown daughter, and in the very first episode of the series called “Sidetracked,” his father (played by David Warner) has been diagnosed with Alzheimer’s disease and is on a downward slope.

When the new season opens in an episode called “The White Lioness,” Wallander is in South Africa and fears he may have an early form of the disease, too. Branagh says he talked with experts about the ways in which people dealing with dementia either hide it from themselves or loved ones.

“That would be particularly frustrating for a man like Wallander,” observes Branagh, “who has to face the fact that his own particular isolationism makes things pretty tricky if you are starting to become forgetful.”

The Swedish landscape — sometimes equally beautiful and desolate — is an important part of the series, which powerfully takes advantage of the imagery. Many of the stories are set in summer when the sun barely sets and there is an eeriness to the light. Yet those familiar with “Wallander” know that Mankell’s mysteries were rarely confined to the parochial but usually touch on larger issues, such as drugs or human trafficking.

The author had a lifelong relationship with Africa, spending a lot of time there and involving himself with numerous charities.

“Henning was a very engaging guy, very outspoken, very provocative,” says Branagh. “But he was very serious about his writing. He knew he was handling magic in that way; so he was very, very careful with his energies. When the muse was on, he grabbed it.”

Branagh says Mankell invited him to interpret Wallander his way.

“He enjoyed it, but said it was not how he imagined it, but he liked it very much,” relates the actor. “He would offer editing notes but not much. He understood that his work was best kept alive by other people bringing their imaginations to it and surprising him.”

A product of London’s Royal Academy Of Dramatic Art, the Irish-born Branagh won a BAFTA in 2009 for portraying Wallander and has been nominated for five Oscars over the years, including twice for his 1989 directorial debut, “Henry V.”

“Wallander” has always been a highly crafted series, which is why fans savor each one. (The first three seasons are available on Netflix.) Tom Hiddleston played the detective’s assistant for the first two years. Later, when Branagh directed the Marvel movie “Thor,” he cast Hiddleston as the villain Loki, propelling the young actor to stardom.

That first “Wallander” episode also featured Nicholas Hoult, who stars in the summer films “X-Men: Apocalypse” and “Equals” with Kristen Stewart.

For Branagh, it has been “a treat” to take his time with the character, who has battled drink, depression and diabetes over the years.

“It’s unhurried, and he’s unhurried,” he says. “Except now poignantly at the end, circumstances are hurrying him, and he’s forced to think about what that means.”

The final episode, which airs May 22, is based on Mankell’s last Wallander novel, “The Troubled Man.” In it, the detective tries to keep ahead of Alzheimer’s while on a case that could threaten his daughter and granddaughter.

Doing the final episodes of “Wallander” were bittersweet for Branagh, but he says Mankell was unsentimental about the story, even when he wrote the book in 2009.

“He took the rough with the smooth, and he didn’t wallow in Wallander’s misfortune, but he was unsparing in the way he presented it.”

Recently, Branagh did a production of Shakespeare’s “The Winter’s Tale” in the West End with Judi Dench that was screened in movie theaters for one day around the world. He’s also developing “Artemis Fowl,” the fantasy books about a teen criminal mastermind, for the screen. So he has no shortage of projects to turn to. Still, he will miss playing Wallander, who “was almost too intense” at times. He’s also grateful for the many great collaborators he has worked with on the show along the way. (Benjamin Carson, who helmed the final “Wallander” episodes, also directed the cinematic broadcast of “The Winter’s Tale.”)

“I’m 55 now. So I’ve been lucky enough to work and acquire some experience, and I’m enjoying being at a point in your life when you can somehow see where that experience starts to pay off. You see how far you have to go to do things. But it’s a nice place to be, because in many ways you’re more relaxed.”

Relaxed. That was something you would never call Wallander.