The presenter discovers how the Thames works around the clock. He explores the stretch of the waterway from the estuary - where the river meets the sea - to the Thames Barrier. It's here, at the Barrier itself, where he begins his journey, as he's given an invitation to its control room to see how it keeps London safe throughout the night.

Tony reveals how the Thames plays a vital role in keeping London - and beyond - on the move, and how it is a 24/7 operation - when the transport network is so busy during the day, it's only at night that crucial maintenance and repairs can be done. His first night shift is with the engineers and cleaners at Thames Clippers - responsible for turning around a whole fleet of commuter boats overnight, ready for the next morning.