Michael Portillo is invited aboard the construction locomotive for Crossrail to travel under the Thames and to meet Mary, on whom the project depends. He travels on the capital's first railway, and admires the remarkable brick viaduct on which it was built. He takes a tour underneath its arches with a Victorian map showing the poverty of those who once lived there. The Docklands Light Railway takes him to Greenwich, home to Britain's most famous tea clipper. And in Woolwich, he discovers the firepower of the British Empire before coming to a sticky end at West Silverton.