Judge Robert Rinder was reduced to tears after discovering his ancestors were killed during the Holocaust on "Who Do You Think You Are?".
The 39-year-old legal expert travelled to Piotrkow, Poland, for an upcoming episode of the genealogy series to trace the steps of his Jewish grandfather Morris, and broke down when he saw a train pass a place where his great-grandparents and five of their children would have been taken to their deaths.
He said: "Just the most staggering thing of passing on a speeding train, your family gone and then you go back to your house and you're alone.
"It's impossible to fathom for my grandfather what that must have been like -- to come back to nothing."
Robert's grandfather was later sent to a concentration camp in Germany, where he was forced to produce missiles for the country's army. At the end of the war, he was brought to the UK with hundreds of other orphans, and made a new life in the Lake District.
And BBC insiders have promised the show will be fascinating for viewers.
The insider told The Sun's Bizarre TV column: "Robert already had some knowledge about his family's history but discovered the full extent of their treatment during filming.
"The programme is already looking like one of the most intriguing yet."
As well as the former "Strictly Come Dancing" contestant, the next series of "Who Do You Think You Are?" will also feature the likes of "Broadchurch" star Olivia Colman, comic Lee Mack, former "Coronation Street" actress Michelle Keegan and pop legend Boy George explore their family histories.