Behind This Great Man Is a Woman : * In ‘221 B Baker Street,’ Sherlock Holmes is a female detective who hides her true sexual identity.

Therese Lentz loves Sherlock Holmes. She especially loves Billy Wilder’s 1970 film “The Private Life of Sherlock Holmes.” She loves the character so much, in fact, that she decided to turn Holmes into a woman.

The result is “221 B Baker Street”--currently in previews at Group Repertory Theatre--in which the famous detective solves crimes while concealing her true sexual identity. “It’s vital to her existence that 19th-Century England not find out she’s a woman,” says the playwright, 31, who’s also producing the show and understudying the lead role. “At that time in history, they were starving women in prisons because they wanted to vote. Women were pretty things that sat in a corner.”

Not this woman. The eight-character story finds “Mr. Holmes” on the case of a young man whose aunt has committed suicide the night before, changing her will at the last moment and leaving her entire estate to her servants.

“I love the play’s sense of humor,” says director Patricia Lee Willson, who is segueing to this show from her recent acting turn in “I Never Sang For My Father” at the theater.

Willson, who also counts herself as a Sherlock Holmes fan, directed the play’s first reading three years ago at GRT and has been along for the long ride. “Therese kept rewriting, and it kept getting funnier,” Willson says. “I’d describe it as a comedy with a lot of plot, because there is a mystery--and just a smidge of ‘Victorian- woman- has- to- pretend- she’s- a- man- to- have- a- great- career.’ ”

Lentz admits that she’s been sitting on this story for more than a decade; when she first came up with the gender-bender slant, it was the time of “Tootsie” and “Victor / Victoria.”

“I let the idea float in my brain for a while,” she says of “Baker Street,” which will mark her full-length, main stage writing debut. “It took a year to get the first draft out.”

Since then, there have been other distractions: playing mom to 4-year-old Sara, and working full-time as a junior high school substitute teacher.

Born in Illinois and raised in Arizona, Lentz graduated from Occidental College as a theater major, with a performance emphasis. “The idea was you need to do everything,” she says. “So I put myself through school building sets, designing costumes.”

It was in school that she also wrote her first one-act, the real-life story of how her parents got married one day after they met. “Acting is still my first love,” she says, “although writing is probably the way I can making a living.”

Lentz is confident that theater audiences won’t have any problem making the leap of faith to Lady Sherlock. “Shakespeare always had women disguised as men to protect them,” she points out. “If you go with the idea of her being a woman, a lot of the (character’s) eccentricities begin to make sense. All you have to say is ‘What if?’ ”

WHERE AND WHEN

What: “221 B Baker Street.”

Location: Group Repertory Theatre, 10900 Burbank Blvd., North Hollywood.

Hours: Opens May 13, and plays at 8 p.m. Fridays and Saturdays, 3 p.m. Sundays. Closes June 19.

Price: Previews are free. After opening, $12 general admission, $10 students and seniors.

Call: (818) 769-PLAY.