Plot: Andrew Graham-Dixon examines the variety of ways in which drawing intimately expresses the creative mind, from the very beginnings of art, the drawings on the ceiling of the Altamira caves in Spain, to the remarkable works made by autistic twins William and Richard Tyler. He also visits one of the rarest masterpieces of the Renaissance, a vast series of pictures uncovered as a result of World War II bombing in Pisa and shows how Picasso influenced a whole generation of artists