Marcia! Marcia! Marcia!

Brady Bunch” star Maureen McCormick - who played the TV family’s oldest girl Marcia Brady - will reveal in a new tell-all book how she had a steamy on-set sexual relationship with the co-star who played her younger sister Jan. The book, titled “Here’s the Story,” will detail how a small crush blossomed into a romantic and physically intimate relationship between McCormick and Eve Plumb, according to published reports. Due out next year on HarperCollins imprint William Morrow, the bombshell biography will also touch on topics such as McCormick’s battles with bulimia, cocaine addiction and depression.

But what will likely shock most fans of the 1970s hit TV series are the details of the budding love affair between the two co-stars who played the highly competitive sisters. It’s not the first time salacious details of behind-the-scenes sex have emerged in the years since the campy classic went off the air. In 1992, co-star Barry Williams, who played Greg Brady, wrote in his tell-all tome, “Growing Up Brady,” how he dated not only his on-screen mom, Florence Henderson, but engaged in several romps with McCormick as well. And here I thought they were just playing Ping Golf all this time.