The presenter watches a tunnelling machine being transported up the river into central London, and discovers plans to build a new tunnel under the Thames to take pressure off the crossing at Dartford. He also makes a late-night visit to New Covent Garden Market, and meets a Paralympian in training at Marlow Rowing Club.

In this episode, Tony travels as far as the Estuary in the East and Waterhay in the West.

The actor and presenter starts the latest leg of his journey in Beckton. There he discovers how a new super-sewer is being built to help stop overflow from London's existing system ending up in the Thames. After a hair-rising cage ride down into the tunnel, Tony Robinson meets the engineer in charge of the project who explains how it will work.