Laura Linney has signed on to play Sherlock Holmes' housekeeper in a new movie about the literary detective as an old man.

Sir Ian MCKellen will play the aged Holmes in director Bill Condon's A Slight Trick of the Mind, and now Linney has been cast as his doting caretaker, Mrs. Munro.

The Truman Show star is a lifelong fan of author Arthur Conan Doyle's Sherlock Holmes stories, and admits she jumped at the chance to be part of the sleuth's world.

She tells, "I was obsessed with Sherlock Holmes as a young kid. You know how some people are into Dungeons & Dragons? I was into Sherlock Holmes. I loved the atmosphere of the stories. I loved the intrigue, his personality. Bill (Condon) had no idea (when he offered me the part)."

The film, which will begin shooting in London and Sussex, England, in July (14), reteams Condon and Linney, who worked together on acclaimed 2003 drama Kinsey and last year's (13) The Fifth Estate. It will also be a reunion for Condon and MCKellen, who worked together on 1998's Gods & Monsters. The project will be Linney's first since she became a mother earlier this year (14).

A Slight Trick of the Mind will be based on Mitch Cullin's 2005 novel about Holmes' later years as the retired detective battles old age and dementia, while trying to figure out one unsolved case.

MCKellen joins the ranks of the stars currently portraying Sherlock Holmes - Jonny Lee Miller and Benedict Cumberbatch play modern versions of the sleuth on Tv and Robert Downey, Jr. took on the character in two Guy Ritchie movies.