On patrol with the Metropolitan Police Marine Unit, Tony gets an emergency call-out to an unexploded bomb. At Billingsgate Fish Market, he relives his teenage years working the docks.

Reaching central London, Tony Robinson boards an industrial working ship, explores the Physic Gardens, and gets a bird's-eye view of the river from the Shard.

Through the beautiful Thames valley, Tony ends up at Hampton Court Palace before trying out a Victorian skiff and exploring the river's most advanced sewage plant.

Tony begins his journey at the source of the Thames in Gloucestershire. He meets a professional crayfish catcher, investigates the cleanliness of the water and discovers the uses the Home Guard put the river to in WWII.