The actress, who is known for playing Mrs. Patmore in the beloved period drama, will appear in episode four as recent widower Irene Moore, who is advised by Doc Martin to stop cold water swimming until the source of her migraines is found.
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In a clip released to RadioTimes.com, Irene can be seen telling Selina Cadell's Mrs Tishell that she feels closest to her late husband when swimming.
Elsewhere in the episode, fans can expect to see Martin run into former girlfriend Sophie Trent, played by Hermione Norris, who invites him to speak at Imperial College in London.
The episode synopsis reads: "Martin rubs up against the new herbalist and Penhale, Louisa and James are all interested in Janice's new pedicure fish. Martin's ex-flame invites him to speak as keynote speaker at Imperial College, and Ruth is back from Mexico with more than maracas."
Lesley's role in the long-running drama comes just four months after the death of her husband, David Heald, was announced.
Lesley plays Irene Moore in the upcoming episode
The 68-year-old's agent, Paul Pearson, confirmed the news in a statement released in May. "I would like to confirm the passing of Lesley's beloved husband David Heald, known to all his wonderful friends as Da'aboth," it read.
"Da'aboth was an amazing, unique and inspirational man who was always at Lesley's side 'her warrior'."
Lesley met the spiritual healer when she was 50 and the pair went on to marry in 2007.
Lesley with her late husband Paul Pearson
Speaking about meeting her husband during an interview with The Guardian back in 2016, she said: "I didn't see him as a potential husband. He’d had an unhappy breakup and was so low he was almost invisible. Slowly, I got to know him and saw what a sweetheart he is. I haven't always had the pleasure of that in relationships.
"There are men who are game players – they can screw with your head. Darbs wouldn't know a game if it bit him on the nose. He’s safe in his own skin."
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