Details on Princess Diana and her stepmother, Raine Spencer, unearthed in new documentary (Exclusive)

A revealing new documentary is shedding light on Princess Diana's contentious stepmother, Raine Spencer.

"Princess Diana's 'Wicked' Stepmother," which premieres on the Smithsonian Channel on Monday, October 28, at 8 p.m. EST, spotlights Diana and Raine's bitter rivalry that comes to take a surprising turn later in their lives.

In a clip from the documentary available exclusively on AOL, which you can watch at the top of the page, royal experts and other insiders give insights into the mysterious People's Princes and her complicated relationship with Raine.

"She was bitterly unhappy. She was disappointed in her marriage, as was Charles. It was a car crash from the very beginning and Diana used to take herself back home to Althorp and hide herself away in her room but not see anybody," royals biographer Penny Junor says in the film. "She was just really miserable. Poor Diana, she didn't have a solid family. She didn't have a mother, Althorp clearly was home."

While her mother moved back to Scotland and remarried, her father married Raine Legge in 1976. Soon thereafter, the woman deemed "Acid Raine" by Diana began to sell off the "treasure trove" of artworks and fineries at the Spencer estate "in a secretive manner," as "Raine & Johnnie" author Angela Levin explains in the AOL exclusive clip.

"Princess Diana's 'Wicked' Stepmother" premieres on the Smithsonian Channel on Monday, October 28, at 8 p.m. EST