Hugh Doherty has spent a lifetime tunnelling underground, from the Clyde River Tunnel to the Hong Kong Mass Transit System. But now he is facing his biggest challenge yet - to build a new tube line under the heart of London, in record time. To speed up the job he is using a spray-on, rapid setting concrete lining, but as another tunnel using the same method collapses, the work grinds to a halt. When his two trusted tunnelling machines then break down, it seems his quest may be doomed ...