This episode follows engineers building London's brand new underground railway, Crossrail, as they take on a Great British obsession - building train tunnels underwater.Linda Miller, an engineer more at home building space launch complexes at Cape Canaveral, battles to rebuild a Victorian tunnel under the Royal Docks in London's East End. Meanwhile, a British tunnelling dynasty - the Bermingham family - follow in the footsteps of the original father-son team of Marc and Isambard Kingdom Brunel to build a brand new tunnel under the River Thames.