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.

The latest leg of the presenter's journey begins in Greenwich, with behind-the-scenes access to the Royal Observatory on the banks of the Thames. Tony discovers the vital role the observatory played in helping sailors navigate the globe. And he has a unique opportunity to view the planet Mars as the Victorians would have seen it - through Britain's largest refracting telescope.

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.

The presenter is on the West London stretch of the Thames, and here he discovers how night-shift workers contend with the river's tides. Tony joins the fire service based at Lambeth, who deal with emergencies on the river, as they practice rescuing a body from the freezing waters in the pitch black.