EXCLUSIVE: Courteney Cox uncovers shocking royal family drama in 'Who Do You Think You Are?' premiere

Courteney Cox may have uncovered a long-lost family secret.

On Sunday's season seven premiere of Who Do You Think You Are?, the Friends star goes back to Europe to trace her maternal line, but what she learns is far more intriguing than she could have expected.

In ET's exclusive sneak peek from the episode, Cox visits the same castle her 18 times great-grandfather lived in the Middle Ages and inquires about a letter she had been told he had sent to the king saying his father had died.

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According to an original 700-year-old financial account, written in Latin and translated to English, Cox finds out that the king's father was still alive and not dead. "I'm confused. Why isn't the father of the king, the king, if he's still alive?" the 52-year-old actress said, befuddled.

In another manuscript from the 14th century detailing items purchased, Cox learns that the father of the king was living in a chamber. "For the bolts, rods, and other ironwork bought for the chamber of the father of the king," Cox reads a translation. "Was that like a prison?"

When it was confirmed that the father of the king was being held prisoner, Cox was visibly upset.

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"He's being kept prisoner by his son?" she asks, only it wasn't the son who was the culprit, but Cox's 18 times great-grandfather.

"My 18 times great-grandfather, Thomas Lord Berkeley, is holding the king's father prisoner," Cox concludes, her interest piqued even more. "Well, this is getting juicy." Sure sounds like a scandalous power play! Watch the exclusive video above for more.

This season on Who Do You Think You Are?, executive produced by Lisa Kudrow and Dan Bucatinsky, Cox will be featured alongside Jessica Biel, Julie Bowen, Jennifer Grey, Smokey Robinson, John Stamos, Liv Tyler and Noah Wyle.

Who Do You Think You Are? premieres Sunday at 10 p.m. ET/PT on TLC.