In this final episode Dawkins examines why Darwin's theory remains one of the most controversial ideas in history.

As Darwin set out on the voyage on the Beagle he still believed that god created the world and everything in it. But the evidence he discovered - fossils, patterns of anatomical resemblance, startling similarities of embryos and domestic breeding - demonstrated the truth: that all life forms vary and that some are more likely to reproduce, passing variations on. His wife Emma, however, was deeply religious and Darwin never criticized religion in public but he believed that "science would bring about a gradual illumination of minds".

Today, Dawkins argues, science has the evidence to prove that evolution is true. Modern discovery of the DNA code which links all life has added to the mountain of evidence showing that evolution is a fact. So why, he wonders as he meets creationists in America, is opposition to evolution more aggressive than ever?

Dawkins is also concerned that back in the UK teaching evolution has become a hugely sensitive issue for science teachers: "This is multicultural Britain. And one of its fault lines runs straight through our children's classrooms. How do we reconcile scientific truth with the deeply held convictions that bind religious communities?"

Returning to the school he visited in episode one, Dawkins confronts the science teachers and challenges their view that they "can't get in to the business of knocking down kid's religions and the religions of families." "There really is", he says, "something special about scientific evidence. Science works; planes fly. Magic carpets and broomsticks don't. Gravity isn't a version of the truth; it is the truth. Anybody who doubts it is invited to jump out of a tenth floor window. Evolution too, is reality."

This equivocation, Dawkins says, began with the Church of England who, rather than attack Darwin, embraced him in a "comfortable relativist fudge". So he meets the Archbishop of Canterbury to ask how he reconciles Darwin and the laws of physics with the miracles described in the bible.

Finally, Dawkins travels to meet an old friend, Dan Dennett, who shares many of his own beliefs, to answer the question Darwin himself was confronted with: how can we find solace in a godless world?