Two decades ago Da Vinci Code mania gripped the world. But the story behind the theory that Jesus Christ had a secret bloodline is more surprising than any thriller. Step aside Indiana Jones and Robert Langdon - BBC Paris Correspondent Hugh Schofield heads to the South of France to uncover a forgotten milestone of broadcasting which helped set the template for the modern conspiracy theory.

The Lost Treasure of Jerusalem was a 1972 episode of the BBC history series Chronicle. It sets out the unusual local mystery of Rennes-le-Château - and the charismatic parish priest who somehow funded a major church renovation. What treasure had he uncovered? Written by and featuring the actor-turned writer Henry Lincoln, the programme was a phenomenon.

The idea that the church was decorated with symbols and clues hinting at the origin of the unexplained wealth gripped viewers and led to two follow-up programmes. But Lincoln's research for the programmes became the keystone of the book The Holy Blood and the Holy Grail - popularising theories of Christ's marriage which went stratospheric with the 2003 release of The Da Vinci Code. Intrepid Hugh reveals the forgotten global impact of the Chronicle series - speaking to The Damned drummer Rat Scabies who had a surprising ringside seat for much of the drama, and to Dame Marina Warner who was the star of a thrilling encounter with the three authors whose book was about to become a global best-seller.

We hear how this forgotten series popularised a spurious new approach to historical research and facts - one that reverberates through conspiracy theories today.

Presented by Hugh Schofield Produced by Kevin Core