Historian Tom Holland explores how a new religion - Islam - emerged from the seedbed of the ancient world, and asks what we really know for certain about the rise of Islam.
The result is an extraordinary detective story.
Traditionally, Muslims and non-Muslims alike have believed that Islam was born in the full light of history. But a large number of historians now doubt that presumption, and question much of what Muslim tradition has to tell us about the birth of Islam.
As a result, Tom finds himself embroiled in what, for 40 years now, has been an underground but seismic debate: the issue of whether, as Muslims have always believed, Islam was born fully formed in all its fundamentals, or else evolved gradually, over many years - and in ways that Muslims today might not necessarily recognise.
So who was the historical Muhammad, and where - if not from God - might the Qur'an, the Holy Book of Islam, actually have come from?
By asking these questions, Tom - as a non-Muslim - has no choice, over the course of the film, but to negotiate the fault-line that runs between history and religion, between doubt and faith.