Tim Dunn and Siddy Holloway pack their bags to go on a trip around the sprawling Heathrow Airport underground stations - the first ever underground rail link between an airport and a city. They start their trip at the original Heathrow Central, now Heathrow Terminals 2 and 3, stopping off at Hatton Cross, with its eye-popping 70's mosaic tiling and speedbird logos, before they finally reach the futuristic 80's design of Terminal 4. Finally, they embark on a special trip along the Heathrow loop, the tube tunnel link which passes underneath the runway. Climbing through the driver's cab, they alight at a secret platform to explore a ventilation shaft and emerge above ground to the sound of aircraft.Next, Siddy heads to north west London to delve into Swiss Cottage station, one of the first stations to be built on the expanding Metropolitan Railway. With those original platforms closed to the public 80 years ago, she explores what remains of that forgotten world.At the museum Tim has a ride on an Elizabeth Line train simulator and at the depot he meets contemporary artist Mark Wallinger who created the ubiquitous labyrinths which adorn the walls at 270 Underground stations.