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.

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.