Four and a half thousand years ago, along the rivers of what is now Pakistan and northwest India, people built the largest and most carefully organised cities in the world. Streets laid out on a grid. Fired bricks in the same proportions from the mountains to the sea. Covered drains running beneath the roads, and a well in nearly every courtyard. And then, when you look for the people who must have ordered all of it — no palace, no royal tomb, no monument, no name. Tonight we walk slowly through those swept brick streets at dusk, and sit for a while with a very large, very quiet question that nobody has answered.

This is a long, unhurried episode about the Indus Valley: the Great Bath on its raised platform, the seals with their unicorns and their four or five undeciphered signs, the weights that agree with one another across a thousand kilometres, the toys with wheels that still turn. We also spend time with what actually happened at the end, which is slower and gentler than the old stories suggest — a monsoon thinning over two centuries, villages moving eastward, and a civilisation that stopped being a civilisation without anyone alive ever experiencing it as an ending. Every theory here is held lightly, because the honest position on almost all of it is that we don't know.

There's nothing to follow and nothing to remember. The picture is deliberately dim and the movement deliberately slow — soft drift across old photographs on a black canvas, no music, no sudden sound, nothing that will pull you back once you've started to go. If you drift off in the first ten minutes, the episode has done its job. Rest well.